The Science and Importance of Sleep

A bed on top of a pyramid in the clouds, to illustrate the importance of sleep


Sleep is a critical activity in our mental and physical health. It is probably the single most effective thing you can do to reset your brain and body health. Sufficient sleep is necessary for learning, productivity, and emotional regulation. It boosts our energy, improves our performance, and helps us prevent injury, making it an athletic necessity on par with hydration and nutrient intake.

Sleep deprivation, meanwhile, has been linked to an increased risk of stroke, obesity, cardiovascular disease, and onset dementia. Lack of sleep also impairs reasoning, problem-solving, and attention to detail, among other effects.


Table of Content


The Glymphatic System


For the body our lymphatic system cleans the body of toxins and keeps us healthy. For the brain it is the glymphatic system. The “g” in glymphatic stems from the facts that it is the glial cells in the brain that makes up the glymphatic system.

Without sleep, our glymphatic system does not have the time to properly clean/garbage collect our brain of the toxic proteins beta-amyloid and tau. The garbage collection typically is most active during the deep non-REM sleep stage (see below for the description of the four stages of sleep). Tau protein and beta-amyloid is found to contribute to Alzheimer’s disease, which is one of the reasons you would want to the garbage collection of the brain, through sleep, to work properly. Both the sleep disorders insomnia and sleep apnea, are associated with a high risk of Alzheimer’s pathology, and also the transition to early stages and ultimately the transition to full-blown Alzheimer’s disease.

Illustrates how cerebrospinal fluid (in blue) flows through the brain and clears out toxins through a series of channels that expand during sleep.
Cerebrospinal fluid (blue) flows through the brain and clears out toxins through a series of channels that expand during sleep.
Source: https://www.nih.gov/news-events/nih-research-matters/how-sleep-clears-brain


The Sleep Stages


Stage 1 non-REM (light sleep)

This is the lightest stage of non-REM sleep, where we transition from wakefulness to sleep. In this stage, our brain waves, heartbeat, breathing, and eye movements slow down. Our muscles also relax, with occasional twitching. We usually spend only 5% of your total sleep time in stage 1, which typically lasts several minutes at a time.


Stage 2 non-REM (deeper sleep)

This is a period of light sleep before we enter deeper sleep. Your heartbeat and breathing slow, and muscles relax even further. Our body temperature drops and eye movements stop. Our brain-wave activity slows, but we experience brief bursts of electrical activity, known as sleep spindles. Studies suggest sleep spindles help with memory consolidation. We spend most of our total sleep time in stage 2 (about 45%). This stage typically lasts around 25 min in the first cycle, with time increasing in each cycle.


Stage 3 non-REM (deepest sleep)

This is the deepest stage of sleep, also known as slow-wave sleep (SWS), and accounts for about 25% of total sleep time. It occurs in longer periods during the first half of the night. Our heart rate, breathing, and brain waves slow to their lowest levels, and muscles completely relax. This stage is the hardest to awaken from, and when sleepwalking, bedwetting, and night terrors occur.

In this stage, our body repairs itself, regrows tissues, strengthens the immune system, and builds bone and muscle. We needs this stage of sleep to wake up feeling refreshed.


Stage 4 REM (dreaming)

This is the stage of sleep where dreaming and nightmares mostly occur, although some can also occur in non-REM sleep. It usually begins about 90 min after a person falls asleep. Our eyes move rapidly from side to side with eyelids closed during this stage (hence the name REM = Rapid Eye Movement), and heart rate and breathing increase. Mixed frequency brain wave activity becomes closer to that seen in wakefulness. Our breathing becomes faster and irregular, and our heart rate and blood pressure increase to near waking levels. Actually, during REM some parts of our brain are up to 30% more active than when you are awake(!)

Just before you enter REM sleep, the brainstem (which is were the dynamic non-REM and REM sleep are played out and then expressed in the cortex and in the body) sends out a signal down the spinal cord. This communicates with the alpha motor neurons in the spinal cord, which control voluntarily skeletal muscles. It is a signal of paralysis. This is why your e.g. arm and leg muscles become paralyzed, so as to stop a person from acting out their dreams. In other words, the brain paralyzes the body so that you can dream safely. Experts believe a person needs both some REM and non-REM sleep for memory consolidation.

People spend around 25% of total sleep time in REM sleep, with each cycle lasting from 10 min – 60 min. As we age, we spend less of our time at sleep in the REM stage. This is also confirmed by the fact that REM sleep is also the strongest predictor (among the sleep stages) of longevity (given by a linear relationship).

REM has many benefits e.g. learning and can be looked at as a kind of self-induced overnight therapy, or emotional first-aid. Also, peak levels of testosterone happen during REM sleep, during the second half of the night.


Woman having a high-quality deep sleep in a white room, with white linen, holding onto a pillow tat resembles a cloud.
Dreaming during REM sleep



😴 How Much Sleep Do We Typically Need?

Sleep needs vary from person to person, depending on their age. As we age, we typically require less sleep to function properly. According to the CDC, the breakdown is as follows:

  • Newborns (0–3 months): 14–17 hours
  • Infants (4–12 months): 12–16 hours
  • Toddler (1–2 years): 11–14 hours
  • Preschool (3–5 years): 10–13 hours
  • School age (6–12 years): 9–12 hours
  • Teen (13–18 years): 8–10 hours
  • Adult (18–60 years): 7+ hours
  • Adult (61–64 years): 7–9 hours
  • Adult (65+ years): 7–8 hours


⚙️ Sleep Mechanisms

There are two internal biological mechanisms: the circadian rhythm and Sleep-wake homeostasis.
These mechanisms work together to regulate when we are awake and sleep.  

Circadian Rhythm
Our circadian rhythms direct a wide variety of functions from daily fluctuations in wakefulness to body temperature, metabolism, and the release of hormones. They control our timing of sleep and cause us to be sleepy at night, and our tendency to drift into wakefulness in the morning, without an alarm clock. Our body’s biological clock, which is based on a roughly 24-hour day, controls most circadian rhythms. Circadian rhythms synchronize with environmental cues such as light and temperature about the actual time of day when these occur, but the rhythms continue even in the absence of cues.

A diagram illustrating circadian rhythms, showing the cyclical patterns of melatonin levels and body temperature throughout a 24-hour period, highlighting times of wakefulness and sleep.
Image source: NIH.gov
Our body’s circadian rhythms affect a variety of functions including body temperature (white line). Melatonin (a hormone released by the pineal gland) helps us feel sleepy once there is less light in our surroundings. The peaks & valleys of melatonin (gold line) are important for matching the body’s circadian rhythm to the external cycle of light and darkness.


Sleep-Wake Homeostasis 
The sleep-wake homeostasis keeps track of your need for sleep. The homeostatic sleep drive reminds our body to sleep after a certain time and regulates sleep intensity. This sleep drive gets stronger every hour you are awake (as it builds up), and causes you to sleep longer and more deeply after a period of sleep deprivation.

Factors that influence our sleep-wake need include medical conditions, medications, stress, sleep environment, and what you eat and drink. Perhaps the greatest influence is the exposure to light, in particular sunlight (see below section). Specialized cells in the retinas of your eyes process light and tell the brain whether it is day or night and can advance or delay our sleep-wake cycle. As a result, exposure to light can make it difficult to fall asleep and return to sleep when awakened.

Night shift workers often have trouble falling asleep when they go to bed, and also have trouble staying awake at work because their natural circadian rhythm and sleep-wake cycle is disrupted. In the case of jet lag, circadian rhythms become out of sync with the time of day when people fly to a different time zone, creating a mismatch between their internal clock and the actual clock.


☀️ The Benefits of Sun Exposure


Sunlight Exposure and Hormone Regulation
Sunlight exposure plays a crucial role in regulating hormones, particularly testosterone and estrogen. According to Dr. Andrew Huberman, a study showed that when men and women go outside for 20 min during the first half of the day (ideally 30-40 min), 3 times a week, and try to expose as much of their skin as they possibly could to sunlight, while still being decent, it can significantly increase testosterone and estrogen levels. This is attributed to the skin’s ability to function as an endocrine organ, synthesizing and releasing hormones in response to sunlight exposure.


The Skin as an Endocrine Organ
Traditionally considered a protective barrier, the skin is now recognized as an active endocrine organ. It contains melanocytes, cells that produce melanin, the pigment responsible for skin color, and also vitamin D. Upon exposure to sunlight, melanocytes synthesize vitamin D, which in turn influences hormone production.


Sunlight and Testosterone
Sunlight exposure stimulates the production of testosterone. Testosterone plays a vital role in regulating various bodily functions, including muscle mass, bone density, and sex drive. Studies have shown that regular sunlight exposure can increase testosterone levels by up to 30%.


Sunlight and Estrogen
Sunlight exposure also enhances the production of estrogen. Estrogen is essential for reproductive health, bone health, and cardiovascular health. Studies have demonstrated that regular sunlight exposure can increase estrogen levels in both men and women.


Maximizing Benefits from Sun Exposure

  • Expose skin to sunlight during the first half of the day, ideally between 6:00 AM – 8:00 AM. Sunlight exposure during this time is most effective for hormone production.
  • Expose as much skin as possible, while still maintaining decency, as it allows for greater absorption of sunlight and enhanced hormone synthesis.
  • Limit artificial light exposure at night, particularly from 10:00 PM – 4:00 AM. Artificial light can suppress melatonin production, the hormone responsible for sleep regulation.
  • Morning sunlight coordinates all the cellular and organ systems of your body. This means that it helps to regulate your body’s natural rhythms, which can improve your sleep, energy levels, and mood.
  • Morning sunlight boosts a number of chemicals that need to be released early in the day. These chemicals include cortisol, which helps you to wake up and feel alert, and catecholamines, which give you increased focus and energy.
  • Morning sunlight increases immune system function throughout the day. This is because it helps to produce white blood cells, which are your body’s defense against infection.
  • Morning sunlight synchronizes your circadian rhythm so that melatonin is being released about 16 hours later to make you sleepy and fall asleep easily. This means that getting sunlight in the morning can help you to sleep better at night.

If Your Sleep Schedule is Disrupted
Sleep deprivation, whether due to all-nighters or other factors, can significantly impair brain function and decision-making abilities. During periods of sleep deprivation, the body’s temperature drops to its lowest point, typically occurring about two hours before habitual wake-up time. This temperature nadir (lowest point) coincides with a decline in cognitive function, emotional regulation, and overall mental state.

During this low-temperature phase, the brain operates at a reduced capacity, making it less reliable for making sound judgments or engaging in complex tasks. Decisions made during this period may be impulsive, poorly considered, and prone to errors.

However, despite these limitations, there is a strategy to harness this low-temperature phase to your advantage. By deliberately cultivating positive anticipation about the next day’s events, you can prime your brain for better sleep quality, even if sleep duration is limited.

A study conducted at Stanford University demonstrated the effectiveness of this approach. Participants who engaged in positive anticipation about their upcoming day experienced improved sleep quality, even when their sleep duration was restricted.

Optimizing Sleep Quality with Positive Anticipation

To utilize positive anticipation for better sleep, consider these practices:

  • Before bed, dedicate a few minutes to reflect on the next day’s events. Identify aspects of the day that you are excited about or anticipate with pleasure.
  • Mentally rehearse positive experiences that you expect to have the following day. Vividly imagine these experiences, engaging all your senses.
  • Express gratitude for the positive aspects of your life. This cultivates a mindset of appreciation and optimism, which can further enhance sleep quality.

By incorporating positive anticipation into your pre-sleep routine, you can mitigate the negative effects of sleep deprivation and promote better sleep, even when your sleep schedule is disrupted.


☕ Coffee

As you are awake for longer, adenosine is released in the brain. Our wakefulness areas are being actively shut down by the adenosine, and our “sleepiness brain areas” are being promoted to be more active. In other words, the longer you stay awake, the more adenosine in the system, and the more tired you become.

As caffeine from e.g. coffee comes into your system, it latches on to the adenosine receptors and competes with adenosine, essentially blocking it (basically hijacking the receptors). However, the adenosine is still in circulation in the brain. So, when the caffeine dislodges from the adenosine receptors you get what is often referred to as a “caffeine crash”. The crash is caused by the brain suddenly being hit by communication from the adenosine receptors from not only the adenosine that was in the system while you where under the influence of caffeine, but also by all the adenosine that were added to the system during that same period. In other words, suddenly you feel extremely tired.

Caffeine is called an anxiogenic, meaning that it increases your anxiety. In terms of biological anxiety, this means having your fight or flight branch of the nervous system switched on into a higher gear, as well as parts of you stress chemistry such as cortisol being ramped up by caffeine. This is directly counter to what you need in order to fall asleep. Caffeine also makes your sleep more fragile, meaning you are more likely to wake up during the night, and it will be more challenging to fall back into sleep.

If you drink coffee into the evening, studies have found that caffeine can reduce the amount of deep non-REM sleep, particularly in first two hours of the night, by ~30% (equivalent to being 10-14 years older).

In addition to caffeine, it turns out that the coffee bean itself contains a very healthy dose of antioxidants (in a family called the polyphenols, in which chlorogenic acid is probably the principal one that carry the main benefits, while acrylamide is another one of the interesting antioxidants). Thus, most of the health benefits of coffee is also found in de-caffeinated coffee.

Given all the above unfavorable side effects of the caffeine in coffee, and that the half-life of caffeine is 5-6 hours, most people should stop their coffee intake about 8-10 hours before their typical bedtime to only be left with the positive benefits of coffee.

Read more about the history and introduction of coffee to learn How Coffee Created the Modern World.


😴 Naps

Studies have shown that naps of as little as 17 min can have some quite potent effects on e.g. learning. NASA pioneered research on naps in the 1990s. During their missions, they were experimenting with naps for astronauts, and they found that naps of as little as 26 min improved mission performance by 34%, and improved daytime alertness by 50%. This lead to a widespread nap culture within NASA terrestrial staff during the 1990s.

However, naps can be a double-edged sword for some people. The longer we are awake, the more adenosine sleep pressure builds up. Thus, by taking naps during the day (the brain clears out some of that adenosine), there will be less adenosine sleep pressure when the evening arrives, which can make it more difficult to fall asleep then. For those that are not negatively affected by naps during the day for their ability to fall asleep in the evening, the recommendation is to not do naps closer than 6-7 hours to going to sleep for the night, and to limit the naps at about 20-25 min in duration (to avoid going into the deeper sleep stages).


🍺 Negative Effects of Alcohol on Sleep

  • Since alcohol is a sedative, it will sedate your cortex, and sedation is not sleep. You may loose consciousness quicker, but you are not naturalistically falling asleep quicker.
  • Alcohol will lead to sleep fragmentation during the night.
  • Alcohol blocks REM sleep, and thereby reduce all the benefits associated with REM sleep.
  • A study done on tipsy subjects showed a 50% drop in growth hormone release during alcohol laced sleep at night. Among other things, growth hormone is vital for metabolism, and keeping body fat low.


🍃 Negative Effects of Marijuana on Sleep

  • Tetrahydrocannabinol (THC), the psychoactive component, has been shown to have benefits in terms of reducing the time it takes you to fall asleep for dosages from 2.5 – 20 mg. However, THC seems to block REM sleep, and thereby reduce all the benefits associated with REM sleep. For people that use THC for sleep, there can be a dependency tolerance, meaning you need more to obtain the same perceived sleep benefit. When stopping, you typically get a very severe rebound insomnia (along with anxiety withdrawal). Thus, THC is not recommended as a sleep aid tool.
  • Cannabidiol (CBD), the none psychoactive component, does not seem to be detrimental to sleep in the way that THC is. In low doses (5-10 mg), CBD seem to be promoting wakefulness. Although in high doses (above 25 mg), there seems to be some sedation like effects from it. However, more research is required on the effects to the quality of sleep and sleep architecture caused by CBD.
    • CBD is thermoregulatory and can cool the core body temperature. This could illude to it having some usefulness to the onset of sleep.
    • CBD is a anxiolytic (it can reduce anxiety), and studies have shown that the amygdala is quieted down by CBD.
    • Some recent studies also suggest that CBD could alter the signaling of adenosine, potentially modulating the sensitivity of the brain to adenosine (making the same adenosine appear more potent), thus creating a stronger pressure for sleep.


🧖 Sauna

When you heat the surface of your body, your core body temperature goes down. What happens is that your medial pre-optic nucleus of the hypothalamus responds to the surface of the body being really hot, so it starts initiating a process to cool down the core temperature. This is beneficial to sleep onset, as falling asleep at night requires a 1-3 degree drop in bodily temperature.

When you wake up in the morning you have a 1-3 degree lower bodily temperature. This triggers a cortisol spike, norepinephrine and then you start waking up. Body temperature and the ability to fall asleep, stay asleep, and wake up are very tightly correlated. Your whole circadian rhythm is temperature driven. This means that it also makes sense to pay close attention to bedroom temperature.


🌡️ Bedroom Temperature

It is crucial to find the appropriate night temperature for your body. When the ambient temperature is already warm, the aim should be to ensure your bedroom is cool enough to sleep, generally around 15.5–19.5ºC (60–67º F). Set your thermostat accordingly.

If it is too cold for you, consider using socks to tweak how you feel until you find a temperature that is right for your body. Trying out socks of different thicknesses is an easy variable to change for tweaking heat loss. Ideal temperature is highly individual and a narrow range, so be persistent with your experiments until you find what is right for you.


🥱 Melatonin

Usually as dusk starts approaching (lack of light coming in trough the eyes), that is when melatonin production and release will start to rise. It usually peaks a few hours later when we are about to fall asleep. As a sidenote, melatonin is produced in the pineal gland, and secreted from there. Melatonin essentially tells the brain and body when it is day and time to wake up, and when it is night and time to go to sleep. Thus, melatonin helps with the timing of the onset of sleep, but not with the generation of sleep itself.

Note that melatonin supplementation is not particularly helpful for sleep. A meta-analysis showed that it only decreases the time-period by which you fall asleep by 3.9 min on average, and it will only improve your sleep efficiency by 2.2%. This underscores that melatonin is primarily regulating the timing of your sleep, not sleep generation.

The optimal doses of melatonin supplementation in which you get benefits has been found in studies to be about 0.1 – 0.3 mg, for the populations that they looked into. This is about 10-20+ times less than what typical companies put into their supplements (which is basically a supra physiological dose: far more than what your body expects). This is because melatonin is currently not well regulated as a supplement, so the strength of melatonin that you buy is often unreliable. There is a study that examined over 15 different suppliers, which found that the concentration of melatonin within each pill ranged from 83% less to 478% more than stated levels on the label. Thus, one should preferably avoid melatonin supplementation, and if you do try it be very careful about the dose and timing.


📝 The Practice of Worry-Journals

There is an interesting and useful practice of writing down your worries and concerns on a piece of paper about 1-2 hours prior to going to bed (keeping a worry-journal). This can be a helpful part of the ritual of winding down for the night. It can reduce the amount of rumination that people tend to experience as they lie in bed trying to fall asleep, and often the rumination that people experience if they wake up in the middle of the night. Studies have found that keeping worry-journals decrease the time it takes you to fall asleep by 50%!


❤️ Positive Effects of Sleep on Relationships

For every hour of sleep that a woman gets, her inclination towards becoming sexually interested intimate with her partner increases by 14%. Professor Serena Chen found in her studies that restless nights leads to far more brutal fights in relationships. Poor sleep would predict higher likelihood of relationship conflict, and if you got into a conflict the chances of you resolving it were significantly lower when the parties had not been sleeping well. This is partly because when you have not slept well, your capacity for empathy is reduced. This leads you to take on more of an abrasive stance with your partner, rather than a more agreeable stance.


📄 Tips for Improving Sleep

  • Spend more time outside exposing much skin (within decency) for at least 20 min during the first half of the day, at least 3 times a week.
  • Be more active during the day.
  • Reduce stress through exercise, therapy, or other means.
  • Avoid alcohol 2-3 hours prior to going to bed.
  • Keep a worry-journal (write down worries/concerns), 1-2 hours prior to going to bed.
  • Turn down or off light coming from the ceiling in hours prior to going to bed (to not interrupt the melatonin release).
  • Avoid blue light from e.g. smart phone, laptop, TV about 1 hour before going to bed (to not interrupt the melatonin release).
  • Consider sauna or taking a hot shower prior to bed to decrease core body temperature.
  • Keep bedroom temperature around 15.5–19.5ºC (60–67º F).
  • Go to bed around the same time each night.
  • Meditate (e.g. in bed) before going to sleep, as it will calm your mind and prepare you for sleep.
  • Place positive anticipation/expectation on the next day events.
  • Consider using a sleep mask to block out any light in your bedroom.
  • Consider using ear plugs to block out any unwanted noise.
  • If you wake up during the night avoid the temptation to look at a watch, and keep lights at a minimum if having to go to the bathroom (to not interrupt the melatonin release).
  • Avoid sleeping in when you have had enough sleep.



Looking to further optimize your sleep through supplements and nutrition, check out:
The Best Supplements for High Quality Sleep



Slideshow version of Tips for Improving Sleep



📚 Sources and References


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