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:
When a person falls asleep and wakes up is largely determined by his or her circadian rhythm, a day-night cycle of about 24 hours. Circadian rhythms greatly influence the timing, amount and quality of sleep. More than 20% of Americans are shift workers who work and sleep against their bodies’ natural sleep-wake cycle. While a person’s circadian rhythm can not be ignored or reprogrammed, the cycle can be altered by the timing of things such as naps, exercise, bedtime, travel to a different time zone and exposure to light. The more stable and consistent the cycle is, the better the person sleeps. Disruption of circadian rhythms has even been found to cause mania in people with bipolar disorder.
What is the function of sleep? There is no single purpose. The body does many things in during sleep.
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.
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.
There is a difference between fatigue and sleepiness. Fatigue, sometimes called tiredness, can result from overwork as well as inactivity and unhealthy eating. There are also people who have fatigue from a variety of medical situations (diseases, chemotherapy treatment, etc.) Sleepiness often accompanies fatigue, but they are not the same. The way to recover from fatigue is to rest. The way to recover from sleepiness is to sleep. You can rest without sleeping. Chemicals or drugs like stimulants can counteract both fatigue and sleepiness for a short time.
Ergonomics experts worry about both fatigue and sleepiness on the job as they often occur together and can both contribute to risk of accident or injury.
One problem if you don’t distinguish between fatigue and sleepiness: responding to the wrong signals. People with a history of insomnia might find themselves fatigued and go to bed. They lie awake in frustration. But they aren’t really sleepy, they are fatigued. People attempting to do sleep restriction therapy would do well to be aware of the difference. If you want to go to bed only when sleepy, be sure you actually are sleepy and not just tired.
The "seven deadly sins" formulated by the medieval monks included Sloth. The Bible in Proverbs 6:9 includes the line: "How long will you sleep, O sluggard? When will you arise out of your sleep?" But a more nuanced understanding of sloth sees it as a disinclination to labor or work. This isn't the same as the desire for healthy sleep. On the contrary, a person can't do work without rest periods and no one can operate at top performance without adequate sleep.
The Puritan work ethic can be adhered to and respect still paid to the sleep needs of healthy humans. It is wrong to see sleep as a shameful activity. On the contrary, we want to celebrate sleep as a good in itself.