You have a severe brain disorder.

At least, that's what a mental health professional might say if you told them that roughly one third of the time:

You experience hallucinations (seeing things that are not there)
You become delusional (believing things that couldn’t possibly be true)
You become disorientated (confusion about time, location, and identity)
You are affectively labile (have wildly fluctuating emotions)
You experience amnesia (forgetting experiences)

Indeed, that would seem to describe a very sick person, but as you know that's how you spend roughly 33% of your life: asleep!

Given that when you sleep you are extremely vulnerable to external threats (like getting attacked or killed by a person or an animal), and that every mammal and even every animal ever studied experiences sleep or something like it, researchers have pointed out that it must be essential to health and well-being to have persisted throughout evolution. Sleep absolutely must provide some kind of survival advantage. But what?

Shockingly, we don't know for sure. That's right, science has yet to adequately describe sleep, nor fully understand what it is and how it confers benefits. We have plenty of clues of course, and we'll review them here. But sleep remains, to a large extent, a mystery.

What is sleep?

We know sleep is essential. Experiments have shown that most mammals will die within 1-3 weeks of complete sleep deprivation. Experiments in humans have shown that it is possible (though assuredly unwise) to stay awake for 8-10 days. In 2012 a Chinese man died after staying awake for 11 days in his attempt to watch all of the European football (soccer) championship matches--judge his decision if you like, but at least he gave us some data.

This makes sleep the third most important component we need for life, behind air (can last about ~3 minutes without it) and water (a few days), but ahead of food (several weeks).

While many mysteries about sleep remain, we know that sleep is not a static process, nor is it a period of dormancy. There are, in fact, five stages of sleep, and we cycle through all of them multiple times each night. In each stage the brain and body have different levels of activity, and we know that all are important for good health.

Stage 1 is the lightest stage of sleep, and typically is the first stage. This is usually followed by Stages 2, 3, 4, and finally REM. A full sleep cycle may take 60-90 minutes, and you will experience several (perhaps five to seven) cycles each evening.

Stage 1 accounts for roughly 4-5% of the time you are asleep. This is a very light sleep, and if you wake up during Stage 1, you may not even realize you've been sleeping. During Stage 1 muscle activity begins to slow, and alpha brain waves begin to appear. The head-bob witnessed in many a boring meeting is a brief entry into Stage 1.

Stage 2 sleep is the bulk of sleep time, about half the time you are asleep. The breathing pattern and heart rate slow down, body temperature begins to drop, and alpha waves predominate.

Stage 3 is the beginning of deep sleep, also known as slow wave sleep for the first appearance of delta brain waves. Stage 3 makes up only 4-6% of sleep time, and like Stage 1, it appears to be transitional to the next stage.

Stage 4 is very deep sleep. In Stage 4 breathing is rhythmic, muscle activity is negligible, and brain waves are almost completely delta. About 12-15% of your sleep is in Stage 4. Have you ever been woken up and experienced complete disorientation? You were probably in Stage 4 sleep. Deep sleep is the hardest sleep to wake up from. In deep sleep, your neurons are all firing together, almost like a meditative group mantra, or synchronized clapping, or waves crashing on the beach.

Finally there is REM sleep, which is the acronym for Rapid Eye Movement. About 20-25% of sleep time is REM. If you watch someone sleep and their eyes are moving back and forth under their lids, they are probably in REM sleep. During REM the brainwaves speed up and dreaming occurs, and brain activity resembles that when awake. Your skeletal muscles stay relaxed but your smooth muscles become activated, increasing the heart and breathing rate.

It's helpful to visualize these sleep stages in graphical format. The diagram below shows an example of a typical night's sleep architecture. This is completely fabricated (that is, it is not based on actual subject data), but is representative of how a night of sleep might progress. A few things should be immediately obvious:

A "cycle" of sleep doesn't mean a perfect linear progression through the stages (e.g., 1, 2, 3, 4, REM, 1, 2, 3, 4, REM, etc.), and sleep can skip back and forth between stages.
The majority of deep sleep happens early in the night.
The stages of REM sleep lengthen as the night goes on such that the bulk of REM sleep occurs later in the night.

Interestingly, REM sleep is considered the deepest sleep by some, but very shallow by others. Given that it is relatively easy to wake someone from REM sleep (and they will typically readily recall what they were dreaming about when woken), I have chosen to represent it as lighter sleep in the diagram below (green bars).

It's worth pointing out that a person's sleep architecture will vary considerably from night to night, depending on a large number of variables, including but not limited to wellness, stress level, prior night(s) sleep quality, and presence of intoxicants or stimulants. However, the cyclical nature of sleep is well structured, and each sleep stage is believed to hold a particular role in physiology.

What causes us to fall asleep?

Given that sleep is essential for life, the body is motivated to make sure it happens. There are two primary mechanisms that control the sleep-wake cycle.

The first is your body clock. Yes, your body keeps time. Every single system in your body is regulated on a 24-hour clock, telling when they should or should not be be undertaking a biochemical process. The fancy phrase for this is circadian rhythm, and it is set by a part of your brain known as the hypothalamus. The hypothalamus is pretty dang important and is involved a few things you might care about: body temperature, hunger, thirst, fatigue/energy, blood pressure, digestion, memory, feelings of love/attachment, and more.

A part of the hypothalamus known as the suprachiasmatic nucleus (SCN) controls your body clock. Everyday the SCN tells the hypothalamus--and thereby every cell in your body--what time it is. The SCN does this by (metaphorically) looking at its watch, then at its phone, then at the wall clock, and deciding what time it is. These so-called "time givers" (zeitgebers in German, a word found pretentiously throughout the literature on this subject) include, most importantly, light, but also food intake, exercise, and ambient temperature.

The SCN needs to check the time daily, because the most important time giver (light), changes every day. Depending on your latitude, you'll have roughly a minute more or a minute less time every day, depending on the season. You're basically going through micro-jetlag every day, and the SCN is there to help you adjust.

The SCN's primary sense mechanism for light is through your eyes. There are special receptors in your eyes for daylight, which send signals to the SCN, prompting release of hormones that affect our brain's sleep-wake cycle. White light, as you're probably aware from basic physics, is composed of many different colors visible across a prismatic spectrum. The receptors in your eyes that communicate with the SCN (and thus set the body clock) are particularly sensitive to light waves in the blue spectrum (around 470 nanometers wavelength - see the diagram below). Thus, when the day gets progressively longer, for example, you should get more blue light exposure to your eyes, telling the SCN how to set your body clock.

At night, the SCN receives the signal that light in the blue spectrum is no longer in the sky. This prompts the SCN to communicate with the pineal gland and tell it to produce a hormone called melatonin. Melatonin has many functions, but one of the main ones is to shut down the adrenal glands. The adrenals produce adrenaline, the function of which is to keep you awake and alert and ready for activity, particularly threats from predators. Melatonin also slows down gut motility (preventing bowel movements while sleeping), and is the communicator for setting the body clock among many cell types.

As the night progresses, melatonin levels drop, and the body prepares for awakening. This includes producing cortisol (the unduly maligned, so-called "stress hormone") from the adrenals and orexin from the hypothalamus. Cortisol has many functions, but a primary wake function is to increase blood sugar and prepare the body to metabolize food. Orexin increases hunger, body temp, and wakefulness, and stirs a drive for activity. Basically, your body prepares to get up, move, and eat.

Once the SCN receives a blue light signal in the morning, melatonin production is shut off, and orexin predominates. The cycle repeats itself once the sun goes down again. (It hopefully goes without saying that this is a gross oversimplification.)

The second system in the sleep-wake cycle is sleep pressure. Sleep pressure essentially builds throughout the day, and is released as we sleep. The longer you stay awake, the greater the sleep pressure. This is why taking a brief nap can be helpful--it releases some of the sleep pressure.

While we have a lot of data on circadian rhythm, sleep pressure is not well understood. Currently the best hypothesis is that the primary (but certainly not the only) effector molecule for sleep pressure is adenosine. Adenosine is a natural by-product of expending energy and builds up in the brain as wake-time increases, and is rapidly decreased during sleep. Adenosine has a sedative effect, slowing down those processes typically involved in wakefulness. In fact, caffeine works by blocking the action of adenosine. That's why if you didn't sleep enough you'll be groggy in the morning from excess adenosine in the brain, and a cup of joe can perk you up.

The diagram below plots both circadian rhythm and sleep pressure across the course of two days. Sleep pressure increases as the day goes on, and decreases during sufficient sleep to reach basal levels. You might also note the blip of circadian sleepiness in the middle of the day. Most people feel that a big lunch makes a person tired (and certainly a large meal is a zeitgeber), but all of us naturally have a slow-down in the mid-afternoon. Why would this be? Perhaps we evolved the mid-day slow-down to get us out of the hot sun on the African savannas? We can't say for sure. But don't kick yourself the next time you get a little sleepy mid-afternoon--it's totally natural.

Why do we sleep?

As mentioned above, sleep is still a mystery of human (and animal) biology. However, we do know that sleep is important for four absolutely critical body functions:

Memorizing
Learning
Healing
Growing

Sleep consolidates memory

It's well understood that sleep is essential to turn short-term memories into long-term memories (called consolidation). Short-term memory is just as it sounds, existing within the waking period: "I just read that, so I can remember it just now." However, to remember that piece of information tomorrow, I need to sleep. If, for example, I study all day and all night for a test ("pull an all-nighter"), it's been shown that such studying doesn't lead to long-term memory because of the absence of sleep.

You literally get smarter as you sleep. [Tweet This]

Scientists believe this occurs through dream-replay of the activity or information involved. While not yet demonstrated, it could be that the cells in your brain express certain gene patterns after an experience, and while you sleep these gene expressions are translated into structural proteins that solidify the neuronal pathways representing the long-term memory.

There is compelling support for the replay theory. If rats are run repeatedly through a maze with a reward at the end while their brainwaves are recorded, scientists can study their brain waves while they sleep and watch the rats' brains re-running the maze. They can even determine what part of the maze they are dreaming about!

We use our brains the same way when we dream about an activity as we do when we are actually performing the activity. Human studies have demonstrated that brain scanning subjects can (with the right database) predict what they are dreaming about based on their brain scan, just like the rats.

Interestingly, most replay seems to occur in non-REM sleep, as demonstrated by a study on subjects who played the computer game Tetris. Some were new to the game, others had played before. Every night, within the first hour of trying to fall asleep, the subjects were awoken and asked what they were dreaming about. Unsurprisingly they dreamed of Tetris. Considering they were woken within the first hour of sleep, it's not likely they had entered REM, certainly not extensive REM.

On the other hand, the consolidation of memories associated with fear is more apt to be linked to REM sleep.

Sleep enables learning

If non-REM sleep (Stages 1-4) is important for replay dreams and memory consolidation, what are the vivid and sometimes bizarre dreams of REM sleep for?

Before going any further, I should point out that studies of dreams (and the awake version: consciousness) are fraught with controversy and dispute. However, neuroscientists are gaining ground, and they have some compelling hypotheses.

Consider this: have you ever been working on a difficult problem or challenge in your life, puzzling over it for days on end, only to wake one morning or in the middle of the night to have an "Aha!" moment presenting the solution? If so, you're not alone. There are many famous anecdotes in both the arts and the sciences of this kind of experience: Jack Nicklaus had epiphany about fixing his broken golf swing; the German chemist Agust Kekule dreamed of the structure of benzene; Hungarian physiologist Albert Szent-Gyorgyi dreamed up how to isolate vitamin C; Samuel Taylor Coleridge dreamed the poem Kubla Khan; Stephanie Meyer dreamed the story of Twilight; and Paul McCartney woke with the song Yesterday fully written. I am sure there are many more.

It seems that while our early evening dreams are replays helping to consolidate memories, as the night goes on our brain seeks to make new connections, attempting to pair prior experiences with new memories, generating bizarre--sometimes terrifying, sometimes productive--insights. The lack of constraints of the real, physical world allow our dreaming mind to look for clues and themes that would never be available to our brainpower when awake and responsible for doing things like eating, working, looking for a mate, or avoiding predators.

In fact the ability to dream free-form may be a pivotal part of being human. While we sleep less in total than any other primates, we spend more time (proportionally) in REM. Some researchers even believe that REM sleep is the evolutionary antecedent of consciousness. Also of note: babies experience nearly five times as much REM sleep as adults, which is not surprising given how important it is for a growing human to gain contextual understanding of its world.

REM dreams may also be important to our ongoing survival. While we are physically paralyzed during REM, our hormonal systems and parasympathetic nervous system are still able to react, including experiencing fear, anxiety, and sexual arousal. It may be that our brain is training itself on how to react to those most important situations in human life: sex, social challenges, and existential threats.

Or it may be that dreams are just plain weird.

"Nothing is as boring as other people's dreams."
--from the novel Paper Towns by John Green

Sleep takes out the trash

No matter how healthy, fit, or tough you are, everybody accumulates cellular damage just by living. As your cells respire and metabolize carbohydrates, fats and proteins to generate energy, toxic byproducts occur. These include reactive oxygen species (ROS) and dicarbonyls (DC), both which float around and damage just about whatever they come in contact with, such as DNA and proteins. That damage can make important components of cellular machinery malfunction.

Fortunately, your body has mechanisms to clean up the damage...to take out the garbage, so to speak. These clean-up mechanisms include production of antioxidants and autophagy. Antioxidants (melatonin is one) remove the ROS, and activate DNA repair machinery. Autophagy (which means "self eating") captures and degrades proteins and cellular components damaged by ROS and DC.

These functions are critical for brain health. Removal of ROS reduces oxidative stress, which is one of the main causes of brain cell death. Autophagy protects against neurodegenerative diseases--Alzheimer's, Parkinson's, and Huntington's are all associated with defects in autophagy.

And as you might have guessed, these repair mechanisms are increased during sleep, and largely regulated by melatonin. Interestingly, melatonin decreases with age, which may explain (in part) the increased occurrence of brain and other chronic diseases as we get older.

Sleep also increases extracellular fluid flow in the brain and enhances removal of metabolites and breakdown products from the brain...the garbage truck if you will.

Finally, sleep is a time when the brain prunes neurons. The brain cannot keep and maintain every neuronal synapse it creates, and sleep is the time when the neurons of lesser importance are removed in order to make way for the more important neurons. You didn't really need those old phone numbers, did you?

Sleep brings on the Juice

There are a lot of different performance enhancing drugs (PEDs) that athletes can use, one of which is human growth hormone (HGH). Athletes use it because it helps them recover from their workouts faster by promoting healing and tissue regeneration, it promotes bone growth for resiliency, and it promotes muscle growth for strength and speed.

HGH is, naturally, outlawed in just about every sport except bodybuilding.

However, for a normal person HGH is just as important. Everyone needs tissues that heal, bones that resist breaking, muscles that function well, and immune systems that fight infections. HGH promotes all these functions.

Sleep is when the majority of HGH is released, specifically during the deepest stages.

Deep sleep is restorative sleep. [Tweet This]

What is "normal" sleep?

Let’s get something clear right away: we do not sleep like we were evolved to. No one I know builds a campfire at dusk, cooks dinner over it, and goes to bed shortly thereafter, staying in bed until the sun comes up and waking up naturally at dawn. Yet that’s how we evolved. During the long days and short nights of summer, sleep was less than in the short days and long nights of winter. Our sleep-wake cycle is genetically tied to day length, and naturally should vary dramatically by season.

Nope. Just about everyone I know watches TV or uses electronic devices far past sundown, and uses an alarm clock to get up in the morning before their body is ready to do so. A recent poll by the National Sleep Foundation found that Americans spend an average 6 hours and 45 minutes in bed each night...which means even less actual sleep.

While that might be "normal" right now, it is neither consistent with our biology nor what is recommended by sleep experts.

What is recommended by experts? You know the answer because it's plastered everywhere: 8 hours every night.

The 8 hour recommendation is based on two key facts. First, epidemiological data is fairly clear that if the average person achieves 8 hours of sleep every night, there is essentially no risk of negative repercussions from sleep deprivation. Once the average amount of sleep falls below 7 hours per night (as in the case of, say, 6.75 hours like the average American), negative effects are almost certain to occur. So according to the best data we have, if the average person gets more than 8 hours, that's enough to avoid sleep deprivation.

Second, a series of fascinating experiments demonstrated that normal adults average 8 hours of sleep. Volunteers agreed to be subjected to a stimulation-free, dark environment every night for over 14 hours for up to three months. The participants reported to the sleep lab every night for weeks. In the first days of the experiment, participants slept for shocking amounts of time, from 12 to up to 20 hours! Talk about sleep debt! After several weeks of acclimation, all participants settled into a consistent average sleep time of about 8 hours, although some were as low as 7 and some as high as 9. Because they were basically locked in a dark room for 14 hours, this means that the participants had a lot of time to just lie there. The researchers observed that the participants adopted a biphasic sleep schedule: they would be awake for a few hours at first, then fall asleep for a few hours, wake in the middle of the night for an hour or two, then sleep for several hours again.

Biphasic sleep is actually our natural sleep pattern, and we have not "evolved" out of it, as demonstrated in the sleep isolation study described above. That biphasic pattern mirrors our sleep from the age before electricity. Letters analyzed from that era reveal that people referred to their "first sleep" and "second sleep" of the night. The mid-sleep waking period was a time for the bathroom, or sex, or simply contemplative thought. Recent studies of present-day, non-industrial, hunter-gatherer societies have also confirmed this biphasic pattern. Clearly sleeping through the night for a full 8 hours straight is not "natural," and a good night of sleep typically involves waking up at least briefly after being asleep for a few hours.

In fact, the mid-night wake may be an instrumental part of our biology. Research has shown that levels of prolactin are highly elevated at night, which creates a quiet, meditative state of mind. This peaceful wake-time has been lost to history, but may have been instrumental for our stress control.

I know what you're saying right now: "I don't need 8 hours! I function great on 6 hours!" That may be true...but it would make you exceptionally special. As shown from the sleep isolation studies, there certainly is an inter-personal difference in the amount of sleep needed. However, the vast majority of humans need at least 7 hours of sleep. There certainly are "short sleepers", and you may be one of them...but there are just as many "long sleepers," people who may need more than 10 hours per night! It is exceedingly unlikely that you are among the rare few that 6 hours of sleep is sufficient to maintain all of your important biological functions.

Another area of biological variability is chronotype. Some people naturally tend to wake up early and are more alert earlier during the day (the so-called "morning person"), and some people naturally tend to wake up late and are more alert later in the day (the so-called "night owl"). This also follows a normal distribution across the population, where most fall within averages, say, going to bed 10-11pm and getting up 6-7am, with an extreme few being far to either direction.

Chronotypes also vary by age. The most dramatic example happens at puberty. Teenagers notoriously have a massive chronotype shift, such that they naturally want to stay up and sleep later. This causes all sorts of difficulties because typical high-school schedules don't match the teenage chronotype, which is an egregious and even willful ignorance of basic biology by school administrators. We essentially keep our teens perpetually jetlagged.

Our chronotype shifts in the other direction as we get older. That's the reason the early bird dinner specials are almost exclusively populated by those with grey hair. The elderly get sleepy earlier, sleep less, and wake up in what most middle-age adults (and certainly teens) would think of as the middle of the night. Interestingly, if you plot this out on a graph, you can see that in a hunter-gatherer society, these different chronotypes would mean that at any given time during the night, there was probably always somebody awake, which would be pretty helpful considering all the things that prowl the night trying to kill and/or eat you.

Coming up next in part 2 of the sleep series: what happens if you don't get enough sleep?

Okay, so sleep makes up a big part of our lives and it's important to our health. But how bad is it if we don't get enough sleep? I'm not really slowly killing myself by getting less that seven hours a night, am I?

The occasional night of short sleep isn't really that bad, is it?

Is it?!

Read Part 2 now!

Sleep Series Bibliography