Sleep involves doing literally nothing, lying down and not being conscious. How complicated could it possibly be?
Very. Sleep, the actual workings of sleep, how it happens and what’s going on during it, is something people don’t really think about that often. Logically, it’s very hard to think about sleep while it’s happening, what with the whole “being unconscious” thing. This is a shame because it’s baffled many scientists, and if more people thought about it we might be able to figure it out faster.
To clarify; we still don’t know the purpose of sleep! It’s been observed (if you adopt a fairly loose definition) in almost every other type of animal, even the simplest kinds like nematodes, a basic and common parasitic flatworm.10 Some animals, such as jellyfish and sponges, don’t show any sign of sleeping, but they don’t even have brains so you can’t trust them to do much of anything. But sleep, or at least some regular period of inactivity, is seen in a wide variety of radically different species. Clearly it’s important, with deep evolutionary origins. Aquatic mammals have evolved methods of sleeping with only half the brain at a time because if they slept fully they’d stop swimming, sink and drown. Sleep is so important it outranks “not drowning,” and yet we don’t know why.
There are many existing theories, such as healing. Rats deprived of sleep have been shown to recover much more slowly from wounds and generally don’t live nearly as long as rats that get sufficient sleep.11 An alternative theory is that sleep reduces the signal strength of weak neurological connections to make them easier to remove.12 Another is that sleep facilitates reduction of negative emotions.13
One of the more bizarre theories is that sleep evolved as a means of preserving us from predators.14 A lot of predators are active at night, and humans don’t need 24 hours of activity to sustain themselves, so sleep provides prolonged periods where people are essentially inert, and not giving off the signs and cues that a nocturnal predator could use to find them.
Some may scoff at the cluelessness of modern scientists. Sleep is for rest, where we give our body and brain time to recover and recharge after a day’s exertions. And, yes, if we’ve been doing something particularly exhausting, a prolonged period of inactivity is helpful for letting our systems recover and replenish/rebuild where necessary.
But if sleep is all about resting, why do we almost always sleep for the same length of time whether we’ve spent the day hauling bricks or sitting in our pajamas watching cartoons? Surely, both activities don’t require equivalent recuperation time. And metabolic activity of the body during sleep lowers by only 5 percent to 10 percent. This is only slightly “relaxing”— like dropping from 50 mph to 45 mph while driving because there’s smoke coming from the engine is only slightly helpful.
Exhaustion doesn’t dictate our sleep patterns, which is why people seldom just fall asleep while running a marathon. Rather, the timing and duration of sleep is determined by our body’s circadian rhythms, set by specific internal mechanisms. There’s the pineal gland in the brain that regulates our sleep pattern via secretion of the hormone known as melatonin, which makes us relaxed and sleepy. The pineal gland responds to light levels. The retinas in our eyes detect light and send signals to the pineal gland, and the more signals it receives the less melatonin it releases (although it does still produce it at lower levels). The melatonin levels in our body rise gradually throughout the day, and increase more rapidly when the sun goes down, hence our circadian rhythms are linked to daylight hours so we’re usually alert in the morning and tired at night.
This is the mechanism behind jet-lag. Traveling to another time zone means you are experiencing a completely different schedule of daylight, so you may be experiencing 11 a.m. levels of daylight when your brain thinks it’s 8 p.m. Our sleep cycles are very precisely attuned, and this throwing off of our melatonin levels disrupts them. And it’s harder to “catch up” on sleep than you’d think; your brain and body are tied to the circadian rhythm, so it’s difficult to force sleep at a time when it’s not expected (although not impossible). A few days of the new light schedule and the rhythms are effectively reset.
You might wonder, if our sleep cycle is so sensitive to light levels, why doesn’t artificial light affect them? Well, it does. People’s sleep patterns now have apparently changed wildly in the last few centuries since artificial light became commonplace, and sleep patterns differ depending on culture.15 Cultures with less access to artificial light or different daylight patterns (for example, at higher latitudes) have sleep patterns that have adapted to their circumstances.
Our core body temperature also changes according to similar rhythms, varying between 98.6°F and 96.8°F (which is a big variation for a mammal). It’s highest in the afternoon, then drops as evening approaches. At midway between the highest and lowest points is when we typically go to bed, so we’re asleep when it’s at its lowest, which may explain the human tendency to insulate ourselves with blankets while we sleep; we’re colder then than when we’re awake.
To challenge further the assumption that sleep is all about rest and conserving energy, sleep has been observed in hibernating animals.16 That is, in animals that are already unconscious. Hibernation isn’t the same as sleep; the metabolism and body temperature drop much lower; it lasts longer; it’s closer to a coma really. But hibernating animals regularly enter a sleep state, so they use more energy in order to fall asleep! This idea that sleep is about rest is clearly not the whole story.
This is especially true of the brain, which demonstrates complicated behaviors during sleep. Briefly, there are currently four stages of sleep: rapid-eye-movement sleep (REM) and three non-rapid-eye-movement (NREM) stages (NREM Stage 1, NREM Stage 2 and NREM Stage 3, in a rare example of neuroscientists keeping things simple for the lay person). The three NREM stages are differentiated by the type of activity the brain displays during each.