Every living creature on Earth, provided they live longer than several days, follows a pattern of activity and rest keyed to the circadian cycle of day and night. Every metabolic function in the body is governed by a 24-hour internal clock that regulates all bodily processes.
Sleep would seem a foolish thing for any creature to do. Not only does it leave you open to predation, but it prevents you from doing anything useful: gathering food, protecting the young, building social bonds, finding a mate. The fact that every creature sleeps, and that this has been going on since life began, indicates that sleep serves a vital function that outweighs any possible disadvantage.
In 1952, a graduate student named Eugene Aserinsky was doing sleep studies when he became the first person to prove, using electrodes attached to the head, that the human body goes through an average of five sleep cycles each night, each cycle lasting about 90 minutes, with two phases for each cycle. One phase involved “rapid eye movements,” so he called that REM sleep; the other phase had no eye movement, so he called that NREM sleep, for non-REM sleep. He found that REM sleep always involved dreaming, whereas no dreaming ever occurred during NREM sleep.
The NREM brainwaves are deep slow waves one-tenth the speed of waking brainwaves, while REM sleep mimics the brain when it is awake. Neither of these cycles are similar to meditation, however.
The split between NREM sleep periods and REM sleep periods shifts over the course of the night. The first 90-minute sleep cycle is primarily NREM sleep, with a little REM sleep at the end. However, as the night progresses, REM sleep becomes increasingly dominant, while NREM sleep decreases. The final cycle before waking is largely REM sleep with only a little NREM time. This cycle is present every night in every human. No one knows why.
This means that if a person who normally sleeps until 8:00 am is required to rise at 6:00 am instead, they will miss only 25% of their sleep time, but they will also miss up to 90% of their REM sleep. Conversely, if a person who usually goes to bed at 10:00 pm stays awake until midnight, they will miss a significant percentage of their NREM sleep.
During waking hours, the brain is completely absorbed by processing incoming data. During sleep, however, all data connections are severed. Now the brain can focus on other things. There are parts of our brain that are 30% more active during REM sleep than they are during wakefulness. During REM sleep, the brain appears to be wide awake while the body is sound asleep. Mere seconds before the brain shifts from NREM to REM sleep, the command goes out to paralyze all voluntary movement. The body becomes completely limp, allowing the dreams to play out without fear of damaging the body.
Every creature, including worms and other insects, frogs, fish, sharks, and even bacteria, sleeps. However, as far as we know, only mammals and birds experience REM sleep.
If you deprive a human of sleep, then allow them to catch up on sleep, which form of sleep will be more important as they rebound? Experiments show that the brain prefers NREM sleep above all else—to begin with. However, on the second night, REM sleep becomes more important, a trend that continues for the next several nights.
Humans sleep less than any other primate, which average between ten and 15 hours, depending on the species. Yet humans have a disproportionate amount of REM sleep, devoting up to 25% of sleep time to REM, whereas other primates average only 9%.
Consider that most primates sleep in the tree tops where they are less than totally secure, whereas humans, even in ancient times, slept in caves, often with a fire for protection. It’s much harder to remain in a treetop bower during the total paralysis of REM sleep.
Humans spend more time in REM sleep because we must navigate complex societies using enormous intellectual capacity. REM sleep nourishes these needs.
Before birth, a fetus moves a lot during REM sleep. Most expectant mothers do not realize that when the baby is active, it’s asleep, because the paralyzing facet of REM has not yet developed. Babies in the womb sleep nearly round the clock, and it’s not until the third trimester that they may be awake for short periods.
Two weeks before birth, the baby spends around nine unbroken hours in REM sleep, increasing to 12 hours the week before birth. At no other time during life will a human experience more REM sleep than the week before birth. During this entire period, the infant’s brain is being wired with all the synapses necessary for a lifetime of emotions, memories, intelligence, thoughts, decisions, and actions. However, alcohol disrupts REM sleep for mother and baby alike.
The body manufactures a chemical called adenosine which causes sleepiness. There are receptors in the brain that adenosine latches on to. The longer a person is awake, the more adenosine is manufactured, and the more brain receptors get plugged with it. Over the course of a day, the “sleep pressure” increases until bedtime. During sleep, adenosine levels plunge, and by the time the person awakens, the brain receptors have been cleared. The cycle then begins again.
If you consistently fail to get enough sleep, adenosine is never completely cleared and concentrations remain high. This results in ongoing fatigue until the “sleep debt” is paid.
Caffeine is also able to plug the same receptors in the brain, delaying the urge for sleep. However, the body continues to manufacture adenosine, so when the caffeine wears off, the body suffers an immediate “caffeine crash” as the overload of adenosine rushes in to take its rightful place.
On the other hand, THC and certain cannabinoids encourage the body’s production of adenosine, encouraging sleep.
Throughout the brain, there are specialized cells called glial cells, from the Greek word meaning “glue.” Collectively it’s called the glymphatic system. These cells shrink by 60% during sleep, opening up space throughout the brain. During NREM sleep, the production of cerebrospinal fluid skyrockets, producing up to 20 times more than during the day. This fluid bathes the brain, moving through the open spaces and clearing out metabolic debris, including stress hormones that accumulate over the course of the day. It’s as if wakefulness causes a daily dose of minor brain damage. At the same time, sleep provides neurological sanitation services, repairing all the damage inflicted during the day, like a janitorial crew cleaning up.
