Outside of death, few things unite every member of the animal kingdom quite like sleep. Some animals, like the large hairy armadillo, sleep upwards of 20 hours a day in captivity, while others, like the giraffe, might only clock 10 minutes daily in the wild. But while some animals sleep more than others, almost all animals need some shut eye.
This is fascinating because sleep puts an organism in a very vulnerable position—and yet the ubiquity of the phenomenon across species seems to suggest that the benefits far outweigh its dangers.
And yet the question of exactly how animals, humans in particular, benefit from sleeping remains one of the biggest mysteries in biology today.
"'Why do we sleep?' is the universal question," said Bryce Mander, a neurologist at UC Berkeley's Sleep and Neuroimaging Lab. "There've been a lot of people who have developed theories about why we sleep, but most of those theories are actually really incomplete because sleep impacts just about every organ system in your body. There are a lot of theories, but no one theory has proven correct."
What is sleep?
So if we're not sure why we sleep, what do we know about it?
In the broadest sense, sleep is a regularly occurring shift in consciousness that is marked by a decreased interaction with one's surroundings. Humans have five distinct stages of sleep, which we pass through in multiple cycles throughout the night, each cycle lasting approximately 100 minutes.
The first stage of sleep is marked by slowed eye and muscle movement and dreams comprised of fragmented visual images usually culled from the previous day's experiences. In stage two, eye movement stops entirely and our brain waves begin to slow. In stage three, the slowest brain waves, called delta waves, begin to appear. During stage four, delta waves dominate and there is no eye or muscle movement.
The effects of sleep on human metabolism is so small that if this were really the only role sleep served, it would make more sense for humans to just stay awake and eat more
The final stage of sleep, called Rapid Eye Movement (or REM) sleep is characterized by random, jerky eye movement, fast, shallow breathing, and the temporarily paralysis of one's limbs.
The REM stage of sleep is also when the sleeper will experience their most bizarre and emotive dreams. Although we can dream at any stage of sleep, REM dreams are almost always noted for their strange, associative nature versus the relative mundanity of the dreams experienced during the first four stages, when you're more likely to dream about what happened to you during the previous day. After the REM stage, the sleep cycle repeats, starting over with the first stage.
What happens when we don't sleep?
In addition to our understanding of the sleep cycle, we also know quite a bit about what happens when we don't get enough sleep.
Sleep deprivation can have deadly consequences, as the CDC and other institutions have pointed out, reportedly contributing to a number of notable man-made disasters including the nuclear reactor meltdowns at Three Mile Island and Chernobyl, the Exxon Valdez oil tanker crash, and the chemical plant disaster in Bhopal, India.
Sleep loss also poses significant health risks to the individual: Insufficient sleep has been attributed to increased risk for diabetes, heart attacks, and strokes, in addition to accounting for at least 800 fatal auto accidents in 2013 (the most recent year for which data is available).
The reasons for this widespread sleep deprivation (about one-third of American adults report regularly getting less than the recommended seven to eight hours of sleep per night) are less certain, although it has been variously attributed to family tension, workplace stress, and social jetlag, the shift in sleep time brought about by social obligations.
Although we are pretty confident about what sleep is and what happens if we don't get enough of it, this is not the same as understanding the role sleep plays as a biological function. Does sleep have a primary physiological function, or does it serve many functions? Is it active only within certain systems of the body, or does it occur at the cellular level throughout the body?
Leading theories on why we sleep
Physicians have been pondering these questions for thousands of years, but it wasn't until the advent of the electroencephalogram (EEG) in 1929 that the function of sleep was able to be studied in any rigorous sense. Within seven years of the EEG's invention, the first four stages of sleep were discovered, and while the somnologists were getting a better picture of how we sleep, the question of why has continued to haunt sleep science for the last 70-odd years.
Recharge the battery
"Now, most people think there are a several different functions that sleep is performing," said Mander. "A lot of the early theories focused on the effects of sleep on brain function because sleep is generated in the brain. The common thinking was, 'Well sleep is generated in the brain, so it must be doing something for the brain.' But it's more complicated than that—there are a lot of different functions that sleep serves. It's a great mystery."
One of the earliest theories about sleep was that since we consumed so much energy while we were awake, our body needed to rest. It has been shown that human growth hormone is secreted during deep sleep (stages 3 and 4). This hormone is responsible for repairing tissue that has been damaged during the day, essentially recharging the battery of the human body.
According to Mander, however, this basic restorative theory of sleep has been debunked for being too simplistic. What is more, he argues, the effects of sleep on human metabolism is so small that if this were really the only role sleep served, it would make more sense for humans to just stay awake and eat more, instead of making themselves vulnerable through sleep. (One study estimates the metabolic energy saved by a night of sleep for a 200-pound person to be roughly equivalent to drinking a glass of milk.)
Taking out the trash
Synaptic homeostasis makes more sense to Mander. The basic idea here is that during the day, as you go about learning and experiencing things, your brain is recording these in new synaptic connections. At night, your brain works to only keep the synaptic connections you need and dissolving the inessential connections. This is important because the more you put your synapses to work, the stronger the connection between the cells becomes. As these synaptic connections are strengthened, they take larger and larger amounts of energy to maintain. Thus, by only keeping the important synapse connections and eliminating the unimportant connections, the brain is saving itself energy.
This is similar to more recent research that studies the role of the glymphatic system in the brain. The glymphatic system, discovered in 2012 by a team at the University of Rochester Medical Center, circulates cerebrospinal fluid through the brain and flushes out waste material, much like the lymphatic system, which is responsible for flushing toxins and waste matter from the body.
The team found a 10-fold increase in glymphatic activity in sleeping mice. The study also found that the brain cells of the mice were actually further apart while they were asleep, allowing the cerebrospinal fluid to flush waste matter from the brain more effectively. As URMC neurologist Rashid Deane put it, "that tells us that sleep appears to be significant in helping the brain eliminate unwanted byproducts."
Another trend in the scientific literature about sleep, Mander said, deals with memory consolidation. Previously, it was assumed that the role of sleep was to protect the brain from interfering stimuli and thereby consolidate memories, which were largely encoded during the day. Yet new research suggests that sleep is far from a passive process, and that sleep may best be characterized as an active brain state for optimizing memory consolidation. Memories are consolidated by firing the same group of neurons that fired when the memory was first formed, and this consolidation occurs during slow wave sleep (stages three and four).
Then there's always the chance that Shakespeare was right all along, and we only sleep perchance to dream. Matt Walker, the lead scientist at the Berkeley lab where Mander works, has developed a theory where REM dreaming allows people to process difficult or stressful memories in a relatively stress-free environment. When a stressful event is happening, there is a surge of chemicals in the brain. These chemicals are largely monoamines like serotonin, dopamine, norepinephrine, which are thought to be connected to arousal, emotion, and cognition. The REM dream stage is a low monoaminergic environment, making it ideal for processing past trauma.
"Sleep may be offering you your own personal therapy session in your brain when you're dreaming."
The idea is that by replaying the events in a less stressful environment, the brain is learning to strip away the visceral aspects of the event so that it can learn from it, rather than simply exhibiting a stress response. If this is correct, then this would mean that something has "broken down" in the brain of someone experiencing PTSD—they'll keep experiencing the event in dreams, but will be unable to parse the valuable, learning experience from the visceral stress response.
"Sleep may be offering you your own personal therapy session in your brain when you're dreaming," said Mander. "Freud's particular belief about what those dreams may have meant might not have been accurate, but the idea that dreams have some relevant function to processing events that we have dealt with in the past is probably spot on."
The take away here is that to ask the question of "Why do we sleep" is to open the floodgates for a century and a half's worth of (often contradicting) hypotheses, theories and experimental evidence. Yet as Mander points out, many of these theories and observations should be viewed less as competitors in the race to explain sleep, and more as puzzle pieces coming together to form a holistic theory that accounts for the "why" of sleep.
"There are a lot of different ideas all over the field of why we sleep, but there is no holistic view," said Mander. "I think that's needed and that's something that more work needs to be done in. Sleep fundamentally touches everything. It's one of the most important topics for our society to be talking about."