There are three kinds of sleep: light sleep, deep sleep, and REM sleep. Important physiological functions happen in all three – there is no wasted junk sleep in a healthy night’s sleep. Most middle-aged and older adults would probably prefer to get more deep sleep and less light sleep. A common shift in sleep architecture as we age is a loss of deep sleep and its replacement with light sleep.
One model that is useful is to think of two sleep systems in your body. They must both be asleep for you to be in deep sleep.
System 1 Sleeping
System 1 Waking
System 2 Sleeping
System 2 Waking
Children easily drift into deep sleep. The phrase "sleeping like a baby" more or less refers to a period rich in deep sleep. This is a period of growth and renewal for the body. Even adults experience a surge in growth hormone during their first deep sleep period of the night.
If there is any type of sleep that most middle-aged and older adults crave more of, it is deep sleep. As we pass from young adulthood to middle age, we get less deep sleep and more light sleep. Intuitively, we want more deep sleep and in the morning describe the previous night’s sleep as a good one if it included a lot of deep sleep.
Scientists have yet to determine exactly why people sleep. However, they do know that humans must sleep and, in fact, people can survive longer without food than without sleep. Sleep serves many functions – or more precisely, many things happen while we are asleep.
Scientists have floated many hypotheses on why humans require sleep:
What is the function of sleep? There is no single purpose. The body does many things in during sleep.
Rebecca Reh at Harvard University posits four possible reasons for sleep:
These are high-level reasons. Even this list is too-high level to be of much use in really understanding sleep. It is a framework, but the details have yet to be filled in.
Caltech professor David Prober enumerates four hypotheses.
Whatever the functions are, it is safe to say that NREM and REM sleep have different functions, because they are so different. The characteristic "brain waves" – the EEG readings – are substantially different. REM sleep waves look like waking brain waves (there are minor differences). The skeletal muscles are paralyzed during REM; sleepers can move around during NREM. Memory consolidation and growth hormone release happen in earnest during NREM. Complex cinematic dreams happen in REM. Thermoregulation of the body happens in NREM but not in REM.
"Sleepiness" cannot easily be quantified although there are tests that can be useful in getting some grip on it. Four common tests are used to measure and quantify effects of stimulants and symptoms of disorders.
The two-phase model provides some guidance as to why people get sleepy – duration of prior waking and place in the circadian cycle.
People describe themselves as feeling "refreshed" after sleep, as if their mental fuel tank has been recharged. But it is not clear at a biochemical level what this refreshment means. The brain uses plenty of energy during sleep, so sleep is not analogous to resting a muscle and allowing energy stores to recharge.
Sleep is about cycles. We run through the stages one after another. Waking (stage 0) transitions to NREM sleep – stage 1 followed by stage 2 followed by stage 3 followed by REM sleep (stage R) After REM the brain may briefly wake (maybe for less than a minute) or go directly to stage 1 again. Each cycle lasts about 90 minutes. Deep stage 3 sleep may disappear in the later cycles – when the brain has recovered from its need for deep sleep.
Borrowing from system theory, we can see sleep as an emergent property of populations of local neural networks undergoing state transitions.
This way of looking at sleep is that it is an "emergent property" of some of the brain's neural networks. Emergence is a word used to describe complex systems arise from simpler interactions of small elements. Many properties in organismic and evolutionary biology are emergent, and the concept finds its way to explanation of many phenomena including swarming behavior of insects and the movement of stock prices.
That's why we can speak of a person or animal being asleep or awake, even though there are so many neurons in the brain. When enough sections of the brain are in this sleep-like state, the person can be said to be asleep. Falling asleep is a state shift for the network.
Anatomists have identified cortical columns in the brain. Also called neuronal assemblies, these are theorized to be a basic processing unit of the brain. They periodically flip between states as shown by input-output relationships. The state that is thought to be "sleeping" is when the column generates a bigger response to a stimulus. The reason this state is defined as the sleeping state is that animal studies show when most of the brain’s cortical columns are in this state, the animal is asleep, and when most are in the opposite state the animal is awake.
This is evidence for localized sleeping in the brain and may be the cause of microsleeps, mental slips, and foggy thinking. What we are getting at here is that sleep as a behavior of the brain as a whole is an emergent property that arises when enough of these cortical columns are in the sleep state. The columns communicate with each other and synchronize through electrical and chemical signals. (Chemical signals include neurotransmitters and neuromodulators such as adenosine, glutamate and GABA.) They tend to flip together between waking and sleeping. Not all columns follow in line and there is plenty of evidence for different parts of the brain being in different depths of sleep at any time of the night. But this emergent property model of sleep appears to satisfy observations about sleep behavior.
There is also new evidence that sleep is a time during which the lymphatic system removes metabolic products from the brain and surrounding tissue. Sleep is a time of cranial maintenance.