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Why We Sleep

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 Deep Sleep Light Sleep
System 2 Waking REM Sleep Awake

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:

  • The brain is able to reorder without the inputs it gets while awake.  The brain has a chance to exercise important neuronal connections that might otherwise deteriorate due to lack of activity.
  • Sleep gives the brain an opportunity to reorganize data to help find a solution to problem, process newly learned information and organize and archive memories.
  • Sleep is a time for serious rest. Sleep lowers a person’s metabolic rate and energy consumption. The allostatic load on the body takes a toll and sleep is a respite.
  • The cardiovascular system also gets a break during sleep. Researchers have found that people with normal or high blood pressure experience a 20 to 30% reduction in blood pressure and 10 to 20% reduction in heart rate.
  • During sleep, the body has a chance to replace chemicals and repair muscles, other tissues and aging or dead cells.  Growth hormones are released during deep 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:

  • Recovery – rest for the body, cell growth, housekeeping for body
  • Protection – keeping quiet and still reduces risk from predators
  • Energy regulation – use less energy when asleep
  • Memory consolidation – formation of long-term memories and learning

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.

  • Repair of cell damage caused by life. Small animals with high metabolism sleep much of the day. Large herbivores can get by with only a few hours of sleep a day.
  • Rest. Long period of intense rest. The body’s energy reserves are replenished.
  • Brain and memory reorganization. Synapses are pruned and cleared. That’s why we wake up feeling refreshed and possibly more optimistic.
  • Reinforcing memory and learnings that were formed during the day.

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.

  • Multiple sleep latency test (MSLT) – time to get to sleep
  • Maintenance of Wakefulness Test (MWT) – time to get to sleep
  • Wilkinson addition test – cognitive test
  • Digital symbol substitution – cognitive test

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.

Sleep as an Emergent Property

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.

Difference Between Symptoms and Signs

Medically there is a difference. Signs are observable by outsides, perhaps using technology. High blood pressure, flushed skin, slow reaction times – these are signs. Symptoms are observable and experienced only by the patient subject.

Pain, amnesia, and sleepiness are symptoms. Both signs and symptoms may indicate a disease or illness but they don’t necessarily. Signs and symptoms may derive from different causes, which often makes diagnosis a challenge. These are “non-specific symptoms”.

Sleepiness is also a part of everyday life. It does not by itself indicate a disorder.

Sleepiness is something every human feels as part of normal life, even when healthy. Yet it can also be a symptom, or at least marker, of a disease. What distinguishes between normal sleepiness and a sign or symptom of something else is its frequency and timing. There is also an intensity level to sleepiness. We know subjectively that sometimes we feel the propensity to sleep more strongly than others.

Sleepiness is also a symptom of many illnesses and disorders, not just sleep disorders.

Sleep Deprivation Leads to Injury

Sleep deprivation leads to physical injury. Epidemiologists have shown people of all ages are more likely to get hurt when they haven’t had adequate sleep. Industrial accidents are more common, and drowsy driving car accident rates are higher. An epidemiology study concluded that chronic insomniacs are 2.5 to 4.5 times more likely to have an accident. Older people with sleep deficiency are more likely to experience increased falls and bone breaks.

Drowsy driving accidents claim thousands of lives every year and result in millions of automobile accidents. The more you look into it, the worse insomnia seems. Its effects aren’t just about frustrations under the covers, but extend to the person’s entire life and to society at large. It has long been known that cytokines – biochemical typically thought of as part of the immune system – are involved in regulation of sleep. And scientists have found that even partial sleep loss results in the lower numbers of natural killer (NK) cells in the blood stream and decreased activity of lymphokine-activated killer cells.

Additional Sleep Deprivation Resources:

Education About the Importance of Sleep

We know sleep helps with education – a well-rested brain can more readily learn – but what about education about sleep. Does it work? Does telling people about the importance of sleep, instructions for sleep hygiene, the symptoms of sleep disorders, and the good and bad points of treatments help them understand? How much is getting through and what can we do better?

This question is of interest to public health officials, parents and teachers of children and teens, and even us at Tuck.

Harvard Medical School runs an excellent website at called Understanding Sleep that includes videos. The site does not dumb down the issues, which we appreciate. Serious fans of sleep should take a look.

The American Academy of Sleep Medicine runs Sleep Education. Note that although it contains information about common sleep disorders it is produced by an organization of health care providers and the site seems eager to get you to go to a sleep clinic. The same organization is responsible for this site.

Health Literacy

Public health people worry a lot about health literacy and are forever generating programs to deal with the fact that – they think – people can’t figure out how to use health and medical information. A federal government website says “nearly 9 out of 10 adults have difficulty using the everyday health information that is routinely available.”

Higher costs and lower quality outcomes are said to be the result. This sounds like an exaggeration to us. Compliance with doctors’ orders may not be high and people may not get the right diet and exercise, but that doesn’t mean they are illiterate.

Do people really not know about the importance of sleep? They probably do, but they choose to forgo sleep because of other interests and commitments. Many people feel vaguely guilty about not getting enough sleep, but it’s not a situation that they don’t know sleep is good for them.


CPAP machines require some instruction, and perhaps notoriously, they are underused, although this isn’t because they are difficult to use so much as uncomfortable and inconvenient. The Sleep Apnea Association takes a stand against CPAP machines being directly shipped to the patient, preferring that it go through health professionals who can offer instruction.

Cognitive behavioral therapy, shown to be the best long-term solution for insomnia, requires a lot of instruction and requires the patient to put in mental effort to learn and adopt new habits and attitudes.  Its effectiveness is estimated at 40% to 50%. Failure could be ascribed to laziness, conflict with other events in the patients’ lives, the belief that a pharmacological approach is better, or a general downgrading in the perception that troubled sleep is a problem worth addressing.

Improvement in hygiene is always recommended for people with sleep problems. The lackadaisical attitude too many people take to sleep causes too many to ignore this important part of life.

A test in New Zealand found a formal education program aimed at teens appeared to have some efficacy. A Chinese study found similar results in a program aimed at adult women. Researchers at Brigham and Young Hospital found college students responded to an internet-based sleep learning system.

A Japanese study found a sleep hygiene education program at the IT company helped reduce afternoon sleepiness in workers. The workers in the night shift got not observable benefit from the program. A test of sleep hygiene education in a psychiatric facility found something similar.

Educating the health care providers

Sleep medicine is a branch of medicine, but most sleep disorders are handled by doctors and nurses who are not speficially trained in sleep. The NIH State­of­the­Science Conference Statement called for more education and awareness campaigns aimed at both health care providers and the general public. The conferences and overview articles always call for more education. It’s an easy thing to call for. But doctors, nurses, and the general public already have enough to cram into their brains.

Public Health Efforts

The federal government used to operate a program and website called Star Sleeper directed at children to get them to value sleep. This program ended. No doubt there is plenty of instruction in schools about sleep today, through different programs. We don’t know how effective it is. Some Croatian researchers found leaflets have a little efficacy in educating teens, girls more than boys. There is wide agreement that insomnia costs society serious money.

When he was surgeon general in 2004, Richard Carmona said in a speech “sleep disorders probably represent health literacy in our society at its worst,” although he said that at a conference of sleep medicine researchers and practitioners. So he may have been playing to the crowd and flattering them to some extent.

Advocates for sleep education campaigns point to other successful efforts like the campaign to raise awareness of hypertension in increasing the percentage of Americans who know high blood pressure can be a problem. Everyone agrees that doctors and nurses need to be instructed in the importance of sleep to overall health and in common disorders. Most everyone agrees the general public needs to pay more attention to sleep issues. Whether anyone is willing to put their money and efforts where their mouths are is another question.