Photo credit: Internapcdn.net
Written by Brian Johnson (Liver King)
Us sapiens spend about a third of our lives sleeping… Perhaps we should start paying more attention.
On the hierarchy of health, sleep trumps almost everything. It's not just the low hanging fruit… it’s fruit that's literally laying on the ground that can nourish health metrics more than just about anything. It's the time when body and brain detox, recover, rebuild and restore. How amazing is your sleep? Exactly what I thought!
Our Native American ancestors believed sleep time was not just for physical rest, but also a time where the human spirit enters and communicates with the spirit world. These encounters provide dreams as guidance for their everyday life and nourishes the deepest parts of our being.
This article’s story will take you through 1) What we do in the modern world; the illusion of productivity… 2) How our ancestors lived; simply, rested and in harmony with nature… 3) How modern science supports ancestral sleep traditions…. and 4) I’ll share what you came here for — The Ultimate Ancestral Guide To Better Sleep — in the modern world.
"Which of these is the wisest and happiest--he who labours without ceasing and only obtains, and that with great trouble, enough to live on, or he who rests in comfort and finds all that he needs in the pleasure of hunting and fishing?” — Micmac Chief (1676)
OUR MODERN WORLD IS ADDICTED TO THE ILLUSION OF PRODUCTIVITY
Like a badge of honor, we proudly boast about how busy and productive we are as if it somehow correlates with self worth… we even say the stupidest things, like “I’ll rest when I’m dead...” and “sleep is for the weak.” Ironically, it’s exactly this kind of thinking that’s silently destroying and slowly killing us modern day Sapiens… it’s our lack of rest and our lack of deep, restorative, regenerative sleep that is making us even more weak, more sick (more prone to autoimmune disorders, chronic fatigue and mental health crises), more unhappy and less valuable than our own grandparents.
By no means am I saying that you shouldn’t break your back to work hard to crush your goals (I certainly do!), but there should be a reasonable compromise with attaining your goals and resting your brain and body. It’s beyond absurd that we undermine the value of sleep, and boast about how we can run on just 4 hours of it… you’re not impressing anyone. The fact of the matter is, without proper sleep, those dismissive remarks very well could end up being a self-fulfilling prophecy.
What We Do In The Modern World
In the modern world, we’re addicted to the illusion of productivity so we wake up inside our boxes (our homes) to an alarm clock, we rush through a morning routine, we jump into transport to get to another box (our jobs), do some work, rush back home and binge on screen activities (Netflix, antisocial media, emails, yada, yada, yada) right up until it’s time to climb into bed so that we can hurry up and do it all over again… like a broken record, this is most of the modern world. The problem with this routine is that it leaves us malnourished… it leaves out all the ancestral living practices that our DNA evolved with… things that our DNA still expects today to entrain our circadian rhythms, regulate our sleep / wake cycles and get a proper night of rest.
Despite its importance, most people don’t give sleep the attention that it deserves… the attention that could make a world of difference… the attention that could put back in, what the modern world left out (to return people back to strength, health and happiness).
Sleep is cyclical
Feel free to skip this obligatory section… feel free to scroll straight down to the Ultimate Ancestral Guide To Better Sleep. I’ve included this section for those of us that enjoy geeking out on seemingly ordinary stuff... and because I just couldn’t help myself. Sleep is a behavioral state — we assume a specific posture and remain immobile with a variable threshold for awakening. Humans (and other mammals) exhibit two different types of sleep distinguished from one another by changes in brain activity: rapid eye movement (REM) sleep and slow-wave sleep (SWS).
When we sleep, REM and SWS occur in a predictable and repeating pattern. We start with a descent towards deep SWS, followed by a return to shallow SWS, and concluding with REM sleep before the cycle repeats. Each subsequent cycle results in lesser amounts of and shallower descent into SWS.
The cyclic nature of sleep. 
Brain activity is distinctly different from that of wakefulness during SWS, which represents a time when waking is the most difficult. REM sleep, on the other hand, is nearly the same as observed when awake. Both stages are critical for strengthening the body and mind, leading to health and happiness.
If we don’t sleep, we die.
Sleep is believed to serve many roles in the maintenance of a healthy brain, but its precise purpose or function remains a bit of a mystery.[2,3] The fact that sleep represents a time of extreme vulnerability strongly suggests that whatever functions occur are not only essential for life, but cannot be accomplished when awake.
The importance of sleep is further illustrated by simply looking at what happens to people when you keep them awake. Perceptual distortions, anxiety, irritability, and temporal disorientation start with as little as 1–2 days of sleep loss, followed by complex hallucinations and disordered thinking thereafter. It takes only three days of sleep loss to present with a clinical picture resembling that of psychosis or toxic delirium.[4,5]
If you don’t sleep, you die... at least, we’ve seen this in animal studies.  So yeah, I’d say it’s rather important.
Evolution of sleep
Understanding sleep demands that we look to the natural environment. Yet, most of the extensive body of research to date has been conducted in non-naive, artificial laboratory settings. This provides ample opportunities to understand sleep biology, but also introduces ample challenges to understand how selective pressures shape sleep.
It therefore makes sense to look to modern-day indigenous tribes for answers about ancestral sleep behavior, since they remain relatively untouched by an industrialized environment that could introduce new selective forces and associated responses that are shaping our sleep. After all, the modern world in highly industrialized regions not only allows sleep to take place in highly protected environments, but also highly dangerous environments too (think EMFs, non-grounded sleep environment, ambient blue light, toxic off-gassing mattresses, and more). As such, we might demand different amounts of sleep.
Believe it or not, sleep patterns entertained by indigenous societies had not been directly investigated until only recently, with mere anecdotes serving as the base of evidence until Gandhi Yetish and colleagues published their seminal paper, Natural sleep and its seasonal variations in three pre-industrial societies, in 2015.
Yetish et al. examined sleep duration and timing relative to seasonal variations in natural light and temperature among three indigenous societies: the Hadza, the Kalahari San, and the Tsimané. They focused on adults and ultimately provide insight based on 94 tribesmen and 1165 days of data.
Several other papers have been published since then, although none have as large a sample size or provide as much detail as Yetish’s original work. Their collective findings are discussed next to give us an idea of natural sleep patterns, which will also be compared to modern habits.
Yetish et al found that the tribesmen averaged a sleep time of 5.7–7.1 hours, and total sleep time was strongly related to whenever they got around to going to sleep. In other words, going to sleep earlier was linked to sleeping longer. Wake time showed no such association, suggesting that these people don’t compensate or “sleep in” for getting to bed later.
Follow-up research by Yetish and colleagues revealed that there was little to no variation in average sleep duration across days of the week. However, nightly variation in sleep duration varied notably for each individual. Again, this variation was owed to primarily to going to bed at different times rather than awakening at different times.
Additionally, work from another lab focusing on the Hadza found that sleep duration relied on environmental factors. Higher temperatures, a longer period of daytime sun exposure, sleeping inside a hut rather than outside, less nighttime light exposure or noise, and less nighttime activity predicted longer sleep times. Yet, aspects of the sleeping platform (bed) and use of fire did not affect sleep duration though it may have affected sleep quality.
Sleep wake schedules
Yetish et al found that the tribesmen stayed awake for varying durations after darkness had fallen, about 2.5–4.4 hours after sunset. Small fires were common, but light levels remained below 5 lux (daylight is at least 10,000 lux and a full moon on a clear night is about 1 lux). Awakenings, on the other hand, occurred about an hour before sunrise.
When the researchers evaluated this data alongside solar light levels, sun exposure wasn’t a good predictor of when the tribesmen went to bed or woke up. Rather, they found that sleep occurred during the period of decreasing ambient temperature, and that awakening occurred near the nadir of the daily temperature rhythm.
Sure enough, most indigenous tribes would relax during the night until they fell asleep, which could be linked to changes in their core temperature. Accordingly, the number one explanation for going to sleep later and getting less sleep overall was being physically active into the night... and no, this is not what you're thinking so you can keep that good habit in your pre-sleep routine. Rather instead, these activities included night hunting for nocturally active animals and/or fishing.
My modern-day tribe and I go to bed shortly after sunset (within 2–4 hours) and typically rise with the sun. On average, we clock a whole 8–9 hours while our little savages average 9.5-10.5 hours.
Chronotypes and coverage: GUARDING THE PERIMETER
In 1966, Frederick Snyder proposed the sentinel hypothesis (page 130):
“Man and other animals have learned that under conditions of danger it is safe to sleep only if sentinels are employed to remain vigilant and to report at intervals that all is well.”
Enter the chronotype. If individuals in a group go to sleep early and wake earlier than other individuals, more of the 24-hour day would be covered by vigilant sentinels. The notion of chronotypes is well-established in both humans and other animals. Colloquially, people are known to be morning larks or night owls.
Despite the establishment of chronotypes, its role in the sentinel hypothesis has been investigated only recently. Analysis of Hadza hunter-gatherer sleep behaviors revealed that just over a third of individuals in a group are awake at any given time. Interestingly, chronotype was not predicted by sex nor environment, but was instead a function of age. The researchers therefore proposed that variability in age distribution would have facilitated adaptive sentinel-like behaviour through greater variation in chronotypes.
In other words, it makes total sense that your teenager wants and needs to go to bed later, and wake later. This way, your teen can guard the perimeter while you sleep and visa versa when they sleep. Elders go to sleep even earlier and wake even earlier thus providing comprehensive coverage around the perimeter. I have yet to research the role of man's best friend here but I'd be willing to bet that this is another good reason to keep a k-9 companion.
Our ancestors slept through the night
Historical evidence of preindustrial Europe suggests that we may have slept in a biphasic pattern — two bouts of sleep bridged by up to an hour or more of sedentary activity. Other reports suggest that a biphasic pattern of sleep was not limited to Europe and has been observed among cultures across Africa, Asia, South America, and Australia.
However, these reports are purely anecdotal.
Objective analyses of indigenous tribes have found that a biphasic sleep pattern does not exist.[8,15] Modern hunter-gatherer tribes instead show a distinct sleep-wake pattern characterized by monophasic nighttime sleep supplemented, infrequently, with daytime napping. Yes, this gives us permission to nap… it’s part of our evolutionary right.
These findings square up nicely with 24-hour profiling of hormonal circadian rhythms in healthy adults. Namely, that they exhibit distinct diurnal and nocturnal states — a biological day and night — and exhibit abrupt transitions between the states — a biological dawn and dusk.
The modern-day mismatch
The American Academy of Sleep Medicine and the Sleep Research Society recommend that adults should sleep 7 or more hours per night on a regular basis to promote optimal health. The average American sleeps about 7–7.5 hours, but one-third get less than 7 hours per night.
Unfortunately, sleeping less than 7 hours has been linked to an increased risk of dying from any cause. And if you don’t die, quality of life is likely to suffer, considering that other research has linked short sleep (5–6 hours) to:
An increased risk of high blood pressure and obesity.
An increased risk of developing type II diabetes.[21,22]
Impaired insulin signaling and glucose metabolism.[23–25]
Greater levels of inflammation,  possibly disrupting the blood-brain barrier.
Lower testosterone levels.
Less fat loss and more lean body mass loss when dieting, even when food intake is identical.
But if the average hunter-gatherer get’s less than 7 hours and is relatively free of disease, then could the issue be with sleep quality rather than duration? The modern world is full of phenomena that disrupt our biological clocks and circadian rhythm, even down to a genetic level.
Perhaps these phenomena demand more sleep to compensate — an adaptation to local conditions. After all, the modern world presents a huge environmental buffer that modifies our exposure to light, heat, EMFs, and movement, among other things.
Non-native blue light around the clock
One of the biggest changes humans have experienced in the past century is exposure to artificial blue light, even after the sun has set.
Sunlight is a composure of many light frequencies spanning 200–3,000 nanometers, some of which we are able to see with our naked eyes. This visible spectrum ranges from about 380–720 nanometers. Blue light is at the lower end of this range, starting around 480 nm, and plays a special role in setting our circadian rhythms.
Specifically, it is blue light that suppresses melatonin production, leading to a synchronization of our circadian rhythm to the environment. Normally, blue light would exist only during the daytime. But now we have access to blue light 24-7, and nighttime exposure from modern day light bulbs, electronics and screens (phones, computers, devices, etc) has been shown to negatively affect sleep quality.[34,35] Of course, 9 out of 10 Americans use electronics before bed; some even use electronics in bed!
While not within the scope of this article, blue light’s suppression of melatonin is a bigger issue than sleep or circadian rhythms. Melatonin receptors exist in many organs throughout the body, and alterations in melatonin signaling have been linked to neurological diseases and cancer. As it turns out, melatonin is one of the body’s most powerful antioxidants, able to directly scavenge free radicals, stimulate antioxidant enzymes, and suppress pro-oxidant enzymes.[38,39]
Lack of early morning sun exposure
In the modern world, most people spend their whole lives indoors. A typical work day involves going outside for no more than several minutes to get in a car and drive from building to building. Free-time, work, eating, and even travel are all conducted sheltered from the sun.
Compare this with our ancestors, who are exposed to sunlight for most of the day. Sun exposure is greatest in the morning, peaking around 9:00 a.m., and slowly declines thereafter even though the intensity of the sun continues to grow until the afternoon. Why? Because our ancestors would seek shade and shelter.
Light exposure and activity levels of hunter-gatherers.
So, modern people needn’t spend all their time outdoors, but there is certainly something to exposing yourself to sunlight in the early morning to entrain that internal biological clock. This is going into the guide (see below).
Lack of vitamin A and magnesium
Vitamin A is a generic term thrown around to refer to both true vitamin A (called retinol), which is what the body needs and can be found only in animal foods, and vitamin A precursors (called carotenoids) found in plants. Almost everyone knows about vitamin A’s role in vision, but this little guy does so much more including regulating our genes and setting our circadian rhythms. When blue light from sunshine enters our eyes, vitamin A translates it into a signal that tells our brain it is daytime. When this signal wanes, our brain knows that it is nighttime. This means that real vitamin A, in the form of retinol (from liver), plays an essential role in helping us fall asleep on time, get high quality sleep, sleep long enough, wake up feeling rested, and staying alert and energetic throughout the day.
Similarly, magnesium is an essential mineral with numerous roles in the body. Magnesium is essential for the synthesis of DNA and proteins, for the generation of energy, and for specific actions in different organs such as the eyes. Over 300 enzymes are dependent on magnesium for proper function. This includes those that regulate our circadian rhythms.
Yet, more than half of Americans don’t meet the average requirement for vitamin A nor magnesium through diet. Worse yet, when it comes to vitamin A, a good chunk of most people’s intake comes from the precursor carotenoids, and our ability to convert these guys into real vitamin A is strongly dependent on our genetics.
Of course, the solution is rather simple: eat your liver. Nutrition authorities like Chris Masterjohn (of Mastering Nutrition) and the Weston A Price Foundation (WAPF) recommend 1–2 servings of liver per week, or about 6–8 ounces per week.
As for magnesium, I personally like a transdermal magnesium oil spray and a oral supplement because it's virtually impossible to get enough magnesium from diet alone... This is due to our modern day nutrient depleted soils).
Electromagnetic field (EMF) exposure is all but impossible to avoid in the modern world thanks to ever-advancing technologies, 3 cell phones for every human and a reliance on electricity. It has certainly brought us many conveniences in life, but not without a dire cost.
Our bodies run on electricity — that’s how our nerves communicate! EMFs influence the human body just like they do any other electrical device, stimulating nerves, muscles, and other biological processes. We are only beginning to scratch the surface of the adverse health effects from low-level, long-term exposure to EMFs.
Some individuals report "hypersensitivity" to EMFs, along with complaints of aches and pains, headaches, sleeping disorders, and fatigue. It’s been shown that people with higher daily exposure to EMFs appear to suffer worse sleep, which may be owed to alterations in brain function and/or impaired cell signaling by way of voltage-gated calcium channels (vgcc). Trust me, you don't need wifi, nor orther EMF devices, when you're sleeping.
Remember, Yetish et al demonstrated that hunter-gatherer sleep patterns were linked to changes in core temperature. These changes are largely brought about by environmental cues. When the sun sets, not only do ambient temperatures fall, but melatonin production increases from the lack of blue light. Melatonin promotes sleep in part by lowering core body temperature.
Yet, today people enjoy their air conditioned homes (with good reason!) and relatively constant temperatures. Studies have linked higher core body temperatures at night with some forms of insomnia, and controlled interventions have found that a lowering of core body temperature predicts when we fall asleep and the quality of that sleep.
Nighttime noise isn’t as bad as heat when it comes to disturbing sleep, but it isn’t off the hook. Exposure to noise during sleep can cause awakening, difficulties in falling asleep, and reduced sleep quality.[50,51] For many, this isn’t an issue because the home provides a nice barrier to most noise. But for many others, especially those living in cities, near railroads and airports, or who have an annoying neighbor, noise exposure is something that cannot be overlooked.
Given everything we have discussed, there are a variety of ways (12 and counting) to improve our sleep, to ensure that we optimally support this sacred time when body and brain recover, rebuild, and restore. This guide is how we transition from the modern world to an ancestral night. Enjoy!
Get early morning sun exposure. Sometimes, you hear people say that the best defense is a good offense. Well, one of the best ways to start your night is to make sure you get early morning sun.[52–54] Maximize sun exposure in the early morning and gradually reduce exposure through the day to entrain circadian rhythms. And yes, lose the sunglasses for those early morning sun sessions. The sun's rays are not intense enough during the morning hours to put you at high risk of eye damage, and it's essential to get direct exposure onto your retina to stimulate the cascade of hormonal processes that optimize circadian rhythm. If you experience significant discomfort merely exposing your bare eyes to morning sunshine, you may have an adrenal issue (more on that later). Light boxes can be a helpful substitute for days when this isn’t convenient or possible. Aim for at least 30 minutes of exposure each morning.
Stay moving. Our ancestors relaxed during the day, even took the occasional nap, but they never sat in an office for eight hours straight. Breaking up bouts of sedentariness with short walks or other forms of physical activity does wonders for metabolic health.[55,56] This is especially true if those walks occur soon after eating.[57,58] Setting a step goal can help, with 10,000 steps being a commonly referenced milestone representing a sufficiently active lifestyle. FYI, this represents approximately 4 miles of walking in total.[59,60] This is how I do it: early morning I walk for 20–30 minutes… this is my cup of coffee and my early morning sun exposure. Early afternoon I walk for 20–30 minutes... this is my afternoon nap. After dinner our tribe walks in rain, snow, or shine for 30 minutes.
Connect to the earth. The earth’s surface possesses a limitless supply of electrons that interacts with our internal bioelectrical environment for normal functioning. Our ancestors remained connected to this energy... we can connect to this energy through what is called Earthing (or grounding), which requires that we simply walk barefoot outside or sit and sleep on grounding sheets like we do (Google it). Especially during sleep itself, grounding can improve quality and reduce stress.
Limit blue light at night. Our ancestors routinely stayed awake for a few hours after sunset, with no more than fire light and moon light to guide them. You can do this too, and there are a variety of ways to do so. Assuming that you need to be exposed to electronics, wearing blue-blocking glasses can help immensely.  You can also look for computer programs that reduce blue light emittance and buy light bulbs that put out little to no blue light. Also, use blackout curtains to block out ambient light from the outside.
Pro Tips: Use Himalayan salt lamps at sunset instead of regular lighting. These, along with candles and firelight, fall into the red light category, which is not harmful to circadian rhythm. Wear UVEX safety goggles at least 2 hours before going to bed... this one is seriously a big deal. They cost $9 and they make all the difference. If you're not going to do anything else, do this!
Limit EMFs. We use EMF meters to make sure there are no electrical fields around our sleep environment... we turn off the wifi (actually don't have wifi but you do, so turn it off at night)... put your phone in airplane mode and make sure that there are no electronics (nor lights) anywhere near you. If you'd like to borrow an EMF meter, we keep loaners on hand; send us an email.
Stay cool. Enjoy warmth during the day, but keep your sleep environment between 64 and 68 degrees Fahrenheit. This will help lower your core body temperature and optimize sleep quality. Consider taking a cold shower, or ending your shower with 3 minutes of cold, or cool water, before bed. This will facilitate a lowering of body temperature which can help you transition gracefully into a night of slumber.
Reduce non-native noise exposure. Use earplugs or headphones... I use over ear headphones so that I can use the Calm app on my phone (of course in airplane mode) when I want to relax and drift away. My favorite selection is Mountain Lake, which plays gentle sounds of wildlife and the subtlest of water features. Trust me, these are the sounds that our DNA expects… it just feels right.
Be consistent. Having an irregular sleep pattern messes with circadian rhythms and impairs sleep quality.[64,65] Develop a sleep routine, starting with early morning sun exposure and movement, and stick to it. This is how you entrain and maintain that 24-hour biological clock... before you know it, you'll be waking at the same time every morning without an alarm.
Eat your liver and mind your magnesium. Beef liver is one of the most nutritious foods on the planet… nature's’ ultimate multivitamin, if you will. A higher quality diet is associated with better sleep quality,[66,67] and there is evidence suggesting that what you eat impacts how you sleep.[68,69] Recall that vitamin A translates daylight into a signal that tells the brain it's daytime. When this signal wanes, our brain knows that it is nighttime. This means that real vitamin A, in the form of retinol (from liver), plays an essential role in helping us fall asleep on time, get high quality sleep, sleep long enough, wake up feeling rested, and staying alert and energetic throughout the day. Bottom line, vitamin A and magnesium (required for over 300 enzymes) regulate our circadian rhythms.
Pro Tips: Get sensible sun exposure, love on liver, bone marrow, skin-on and bone-in wild sardines, wild fish eggs and pastured egg yolks and most of your nutritional bases get covered. Don't forget that our depleted soils require modern man to supplement with magnesium (despite how many veggies you swallow) so find a good magnesium oil (we use Ancient Minerals) and oral supplement (like Trace Minerals). Email us for discount codes, free samples and other goodies.
Don’t eat before bed. Finish your last meal 3–4 hours before you go to sleep. Consuming large, regular meals before bed increases susceptibility to weight gain, obesity and DNA mutations. Importantly, this isn’t a major issue with small, nutrient-dense snacks, like liver or a warm cup of homemade bone soup. It's important to note that eating too close to bedtime can harm your health in other ways too... if you consume more calories than your body can immediately use, there will be an excess of free electrons, which back up inside your mitochondria. These electrons are highly reactive and start to leak out of the electron transport chain. These excess electrons wind up prematurely killing the mitochondria, and then wreak further havoc by damaging cell membranes and contributing to DNA mutations. There's compelling evidence to suggest this type of mitochondrial dysfunction is one of the keys to accelerated aging.
Shut your mouth. Breathing through the nose plays an important part in maintaining airway anatomy, sleep quality and detoxing the brain via the glymphatic system.[71,72] If you have ever had a cold or stuffy nose, then you know how much this can interfere with a good night’s rest. What's more, your nose regulates at least 30 physiological processes, including the release of nitric oxide (NO), a bronchodilator and vasodilator that helps lower blood pressure and helps maintain homeostasis in your body.
Pro Tips: Use an all natural nasal spray, elevate your head and tape your mouth shut... they even make special tape just for this.
Sleep on the ground. Our bodies were never meant to be disconnected from the electrons of the Earth, and they were certainly never meant to be cradled in a modern-mattress cast. At least consider building a little bed out of Earthing materials like my tribe does. Haven’t you ever wondered why you feel so good when you go camping? Or why you wake up a stiff Rick at home? Check out Katy Bowman if you don’t believe me; she is a biomechanist and author of the book: Move Your DNA. She sleeps on the ground. She got us sleeping on the ground.
That's it! Thanks for reading. I hope that you learned at least one new thing that could make all the difference. If you know of anything else, please email us so that we can include an UPDATES section with new discoveries.
UPDATES & HONORABLE MENTIONS:
Just say no. Drugs like alcohol and Ambien both affect normal sleep architecture and sleep quality.[73,74] If you're set on having drinks, start earlier and finish earlier so that by bedtime, body and brain can recover and can drift into the deepest, most restorative sleep that our physiology needs. As it relates to the Ambiens and Niquils of the world, you won't need the stuff once you implement this guide on a regular.
Pro Tip: There's nothing wrong with a little alcohol from time to time... especially if it's done right, and earlier in the day! Wife and I have drinks every now and then and it's clear that the benefits of the sensible indulgence outweigh anything else. An interlude with a good drink (a bottle of wine, or a bottle of Everclear) can deepen the experience... it can strengthen the bond... it can be profoundly positive... in context, of course. If interested, I have an alcohol protocol towards the bottom of our "About Us" page.
- Deboer, T. Behavioral and electrophysiological correlates of sleep and sleep homeostasis. Curr. Top. Behav. Neurosci. 25, 1–24 (2015).
- Watson, B. O. & Buzsáki, G. Sleep, Memory & Brain Rhythms. Daedalus 144, 67–82 (2015).
- Frank, M. G. Circadian Regulation of Synaptic Plasticity. Biology 5, (2016).
- Waters, F., Chiu, V., Atkinson, A. & Blom, J. D. Severe Sleep Deprivation Causes Hallucinations and a Gradual Progression Toward Psychosis With Increasing Time Awake. Front. Psychiatry 9, 303 (2018).
- Reeve, S., Sheaves, B. & Freeman, D. The role of sleep dysfunction in the occurrence of delusions and hallucinations: A systematic review. Clin. Psychol. Rev. 42, 96–115 (2015).
- Rechtschaffen, A., Gilliland, M., Bergmann, B. & Winter, J. Physiological correlates of prolonged sleep deprivation in rats. Science 221, 182–184 (1983).
- Bentivoglio, M. & Grassi-Zucconi, G. The Pioneering Experimental Studies on Sleep Deprivation. Sleep 20, 570–576 (1997).
- Yetish, G. et al. Natural sleep and its seasonal variations in three pre-industrial societies. Curr. Biol. 25, 2862–2868 (2015).
- Yetish, G., Kaplan, H. & Gurven, M. Sleep variability and nighttime activity among Tsimane forager-horticulturalists. Am. J. Phys. Anthropol. 166, 590–600 (2018).
- Samson, D. R., Crittenden, A. N., Mabulla, I. A. & Mabulla, A. Z. P. The evolution of human sleep: Technological and cultural innovation associated with sleep-wake regulation among Hadza hunter-gatherers. J. Hum. Evol. 113, 91–102 (2017).
- Snyder, F. Toward an evolutionary theory of dreaming. Am. J. Psychiatry 123, 121–142 (1966).
- Samson, D. R., Crittenden, A. N., Mabulla, I. A., Mabulla, A. Z. P. & Nunn, C. L. Chronotype variation drives night-time sentinel-like behaviour in hunter-gatherers. Proc. Biol. Sci. 284, (2017).
- Ekirch, A. R. Sleep we have lost: pre-industrial slumber in the British Isles. Am. Hist. Rev. 106, 343–386 (2001).
- Ekirch, A. R. Segmented Sleep in Preindustrial Societies. Sleep 39, 715–716 (2016).
- Samson, D. R., Crittenden, A. N., Mabulla, I. A., Mabulla, A. Z. P. & Nunn, C. L. Hadza sleep biology: Evidence for flexible sleep-wake patterns in hunter-gatherers. Am. J. Phys. Anthropol. 162, 573–582 (2017).
- Wehr, T. A., Aeschbach, D. & Duncan, W. C., Jr. Evidence for a biological dawn and dusk in the human circadian timing system. J. Physiol. 535, 937–951 (2001).
- Watson, N. F. et al. Recommended Amount of Sleep for a Healthy Adult: A Joint Consensus Statement of the American Academy of Sleep Medicine and Sleep Research Society. Sleep 38, 843–844 (2015).
- Youngstedt, S. D. et al. Has adult sleep duration declined over the last 50+ years? Sleep Med. Rev. 28, 69–85 (2016).
- Gallicchio, L. & Kalesan, B. Sleep duration and mortality: a systematic review and meta-analysis. J. Sleep Res. 18, 148–158 (2009).
- Grandner, M. A., Chakravorty, S., Perlis, M. L., Oliver, L. & Gurubhagavatula, I. Habitual sleep duration associated with self-reported and objectively determined cardiometabolic risk factors. Sleep Med. 15, 42–50 (2014).
- Cappuccio, F. P., D’Elia, L., Strazzullo, P. & Miller, M. A. Quantity and quality of sleep and incidence of type 2 diabetes: a systematic review and meta-analysis. Diabetes Care 33, 414–420 (2010).
- Anothaisintawee, T., Reutrakul, S., Van Cauter, E. & Thakkinstian, A. Sleep disturbances compared to traditional risk factors for diabetes development: Systematic review and meta-analysis. Sleep Med. Rev. 30, 11–24 (2016).
- Broussard, J. L., Ehrmann, D. A., Van Cauter, E., Tasali, E. & Brady, M. J. Impaired insulin signaling in human adipocytes after experimental sleep restriction: a randomized, crossover study. Ann. Intern. Med. 157, 549–557 (2012).
- Buxton, O. M. et al. Sleep restriction for 1 week reduces insulin sensitivity in healthy men. Diabetes 59, 2126–2133 (2010).
- Donga, E. et al. A single night of partial sleep deprivation induces insulin resistance in multiple metabolic pathways in healthy subjects. J. Clin. Endocrinol. Metab. 95, 2963–2968 (2010).
- Mullington, J. M., Simpson, N. S., Meier-Ewert, H. K. & Haack, M. Sleep loss and inflammation. Best Pract. Res. Clin. Endocrinol. Metab. 24, 775–784 (2010).
- Hurtado-Alvarado, G., Domínguez-Salazar, E., Pavon, L., Velázquez-Moctezuma, J. & Gómez-González, B. Blood-Brain Barrier Disruption Induced by Chronic Sleep Loss: Low-Grade Inflammation May Be the Link. J Immunol Res 2016, 4576012 (2016).
- Leproult, R. & Van Cauter, E. Effect of 1 week of sleep restriction on testosterone levels in young healthy men. JAMA 305, 2173–2174 (2011).
- Nedeltcheva, A. V., Kilkus, J. M., Imperial, J., Schoeller, D. A. & Penev, P. D. Insufficient sleep undermines dietary efforts to reduce adiposity. Ann. Intern. Med. 153, 435–441 (2010).
- Archer, S. N. & Oster, H. How sleep and wakefulness influence circadian rhythmicity: effects of insufficient and mistimed sleep on the animal and human transcriptome. J. Sleep Res. 24, 476–493 (2015).
- Nunn, C. L., Samson, D. R. & Krystal, A. D. Shining evolutionary light on human sleep and sleep disorders. Evol Med Public Health 2016, 227–243 (2016).
- Smolensky, M. H., Sackett-Lundeen, L. L. & Portaluppi, F. Nocturnal light pollution and underexposure to daytime sunlight: Complementary mechanisms of circadian disruption and related diseases. Chronobiol. Int. 32, 1029–1048 (2015).
- West, K. E. et al. Blue light from light-emitting diodes elicits a dose-dependent suppression of melatonin in humans. J. Appl. Physiol. 110, 619–626 (2011).
- Grønli, J. et al. Reading from an iPad or from a book in bed: the impact on human sleep. A randomized controlled crossover trial. Sleep Med. 21, 86–92 (2016).
- Chinoy, E. D., Duffy, J. F. & Czeisler, C. A. Unrestricted evening use of light-emitting tablet computers delays self-selected bedtime and disrupts circadian timing and alertness. Physiol Rep 6, e13692 (2018).
- Gradisar, M. et al. The sleep and technology use of Americans: findings from the National Sleep Foundation’s 2011 Sleep in America poll. J. Clin. Sleep Med. 9, 1291–1299 (2013).
- Pandi-Perumal, S. R. et al. Physiological effects of melatonin: role of melatonin receptors and signal transduction pathways. Prog. Neurobiol. 85, 335–353 (2008).
- Manchester, L. C. et al. Melatonin: an ancient molecule that makes oxygen metabolically tolerable. J. Pineal Res. 59, 403–419 (2015).
- Tan, D.-X., Manchester, L. C., Terron, M. P., Flores, L. J. & Reiter, R. J. One molecule, many derivatives: a never-ending interaction of melatonin with reactive oxygen and nitrogen species? J. Pineal Res. 42, 28–42 (2007).
- Feeney, K. A. et al. Daily magnesium fluxes regulate cellular timekeeping and energy balance. Nature 532, 375–379 (2016).
- Fulgoni, V. L., 3rd, Keast, D. R., Bailey, R. L. & Dwyer, J. Foods, fortificants, and supplements: Where do Americans get their nutrients? J. Nutr. 141, 1847–1854 (2011).
- Weber, D. & Grune, T. The contribution of β-carotene to vitamin A supply of humans. Mol. Nutr. Food Res. 56, 251–258 (2012).
- Borel, P. & Desmarchelier, C. Genetic Variations Associated with Vitamin A Status and Vitamin A Bioavailability. Nutrients 9, (2017).
- Liu, H. et al. Occupational electromagnetic field exposures associated with sleep quality: a cross-sectional study. PLoS One 9, e110825 (2014).
- Zhang, J., Sumich, A. & Wang, G. Y. Acute effects of radiofrequency electromagnetic field emitted by mobile phone on brain function. Bioelectromagnetics 38, 329–338 (2017).
- Dawson, D., Gibbon, S. & Singh, P. The hypothermic effect of melatonin on core body temperature: Is more better? J. Pineal Res. 20, 192–197 (1996).
- Lack, L. C., Gradisar, M., Van Someren, E. J. W., Wright, H. R. & Lushington, K. The relationship between insomnia and body temperatures. Sleep Med. Rev. 12, 307–317 (2008).
- Murphy, P. J. & Campbell, S. S. Nighttime Drop in Body Temperature: A Physiological Trigger for Sleep Onset? Sleep 20, 505–511 (1997).
- Libert, J. P. et al. Relative and Combined Effects of Heat and Noise Exposure on Sleep in Humans. Sleep 14, 24–31 (1991).
- Basner, M. et al. ICBEN review of research on the biological effects of noise 2011-2014. Noise Health 17, 57–82 (2015).
- Hume, K. I., Brink, M. & Basner, M. Effects of environmental noise on sleep. Noise Health 14, 297–302 (2012).
- Fetveit, A. & Bjorvatn, B. The effects of bright-light therapy on actigraphical measured sleep last for several weeks post-treatment. A study in a nursing home population. J. Sleep Res. 13, 153–158 (2004).
- Rosa, J. P. P. et al. Effect of bright light therapy on delayed sleep/wake cycle and reaction time of athletes participating in the Rio 2016 Olympic Games. Chronobiol. Int. 1–9 (2018). doi:10.1080/07420528.2018.1459660
- Richardson, C. et al. A randomised controlled trial of bright light therapy and morning activity for adolescents and young adults with Delayed Sleep-Wake Phase Disorder. Sleep Med. 45, 114–123 (2018).
- Saunders, T. J. et al. The Acute Metabolic and Vascular Impact of Interrupting Prolonged Sitting: A Systematic Review and Meta-Analysis. Sports Med. (2018). doi:10.1007/s40279-018-0963-8
- Benatti, F. B. & Ried-Larsen, M. The Effects of Breaking up Prolonged Sitting Time: A Review of Experimental Studies. Med. Sci. Sports Exerc. 47, 2053–2061 (2015).
- Solomon, T. P. J., Eves, F. F. & Laye, M. J. Targeting Postprandial Hyperglycemia With Physical Activity May Reduce Cardiovascular Disease Risk. But What Should We Do, and When Is the Right Time to Move? Front Cardiovasc Med 5, 99 (2018).
- Erickson, M. L., Jenkins, N. T. & McCully, K. K. Exercise after You Eat: Hitting the Postprandial Glucose Target. Front. Endocrinol. 8, 228 (2017).
- Tudor-Locke, C. et al. How many steps/day are enough? For adults. Int. J. Behav. Nutr. Phys. Act. 8, 79 (2011).
- Tudor-Locke, C., Craig, C. L., Thyfault, J. P. & Spence, J. C. A step-defined sedentary lifestyle index: <5000 steps/day. Appl. Physiol. Nutr. Metab. 38, 100–114 (2013).
- Chevalier, G., Sinatra, S. T., Oschman, J. L., Sokal, K. & Sokal, P. Earthing: health implications of reconnecting the human body to the Earth’s surface electrons. J. Environ. Public Health 2012, 291541 (2012).
- Ghaly, M. & Teplitz, D. The biologic effects of grounding the human body during sleep as measured by cortisol levels and subjective reporting of sleep, pain, and stress. J. Altern. Complement. Med. 10, 767–776 (2004).
- Burkhart, K. & Phelps, J. R. Amber lenses to block blue light and improve sleep: a randomized trial. Chronobiol. Int. 26, 1602–1612 (2009).
- Phillips, A. J. K. et al. Irregular sleep/wake patterns are associated with poorer academic performance and delayed circadian and sleep/wake timing. Sci. Rep. 7, 3216 (2017).
- Taub, J. M. Behavioral and psychophysiological correlates of irregularity in chronic sleep routines. Biol. Psychol. 7, 37–53 (1978).
- Khan, M. K. A., Chu, Y. L., Kirk, S. F. L. & Veugelers, P. J. Are sleep duration and sleep quality associated with diet quality, physical activity, and body weight status? A population-based study of Canadian children. Can. J. Public Health 106, e277–82 (2015).
- Mondin, T. C. et al. Diet quality, dietary patterns and short sleep duration: a cross-sectional population-based study. Eur. J. Nutr. (2018). doi:10.1007/s00394-018-1655-8
- Peuhkuri, K., Sihvola, N. & Korpela, R. Diet promotes sleep duration and quality. Nutr. Res. 32, 309–319 (2012).
- St-Onge, M.-P., Mikic, A. & Pietrolungo, C. E. Effects of Diet on Sleep Quality. Adv. Nutr. 7, 938–949 (2016).
- Kinsey, A. W. & Ormsbee, M. J. The health impact of nighttime eating: old and new perspectives. Nutrients 7, 2648–2662 (2015).
- Meen, E. K. & Chandra, R. K. The role of the nose in sleep-disordered breathing. Am. J. Rhinol. Allergy 27, 213–220 (2013).
- Pevernagie, D. A., De Meyer, M. M. & Claeys, S. Sleep, breathing and the nose. Sleep Med. Rev. 9, 437–451 (2005).
- Thakkar, M. M., Sharma, R. & Sahota, P. Alcohol disrupts sleep homeostasis. Alcohol. 49, 299–310 (2015).
- Kryger, M. H., Steljes, D., Pouliot, Z., Neufeld, H. & Odynski, T. Subjective versus objective evaluation of hypnotic efficacy: experience with zolpidem. Sleep. 14, 399–407 (1991).