Sleep is an important element of human flourishing and well-being, both subjectively and objectively. Sleep is necessary for a healthy and balanced life. We need quantity and quality sleep in order to function well during waking hours. Nearly all physiological and behavioral functions in humans occur on a rhythmic basis, which in turn leads to dramatic diurnal rhythms in human performance capabilities.

Most adults need around eight hours of sleep to function well. Although many people claim they require less, doctors who study sleep find only 10% require significantly more or less sleep. Recent findings indicate that the chronic lack of sleep and untreated sleep disorders may be factors in the development of cardiovascular disease, obesity, and diabetes.

Scientists believe sleep may be a way of recharging the brain. The brain has a chance to shut down and repair neurons and to exercise important neuronal connections that might otherwise deteriorate due to lack of activity. Sleep appears necessary for our nervous systems to work properly. Too little sleep leaves us drowsy and unable to concentrate the next day. It also leads to impaired memory and physical performance and reduced ability to carry out math calculations. If sleep deprivation continues, hallucinations and mood swings may develop. Some experts believe sleep gives neurons used while we are awake a chance to shut down and repair themselves. Without sleep, neurons may become so depleted in energy or so polluted with byproducts of normal cellular activities that they begin to malfunction.

We typically spend more than two hours each night dreaming. Scientists do not know much about how or why we dream. Only during the 1950s when researchers first described rapid eye movement (REM) did scientists begin to carefully study sleep and dreaming. They soon realized that the strange, illogical experiences we call dreams almost always occur during REM sleep. There are typically five stages during a normal night's cycle.

Diagnostic tools, from X rays and electroencephalograms (EEGs) to CAT scans and NMR imaging, revolutionized procedures, diagnoses, and treatments of sleep disorders in the latter half of the 20th century. Likewise, advances are occurring at an unprecedented rate in the 21st century and scientists are learning more and more about sleep all the time.

Scientists still have an inadequate understanding of the physiology of sleep and the pathology of sleep disorders. Further, although neurodegenerative diseases are associated with abnormal sleep, nobody knows if there is a cause-and-effect in place, and if so, which way it runs.

Sleep for Health

According to National Institutes of Health, "sleep and sleep-related problems play a role in a large number of human disorders and affect almost every field of medicine....At least 40 million Americans each year suffer from chronic, long-term sleep disorders, and an additional 20 million experience occasional sleeping problems." Factors to consider in assessing sleep include sleep disturbance, sleep adequacy, somnolence, quantity of sleep, snoring, and awakening short of breath or with a headache.

About one-third of Americans have symptoms of insomnia but less than 10% of those cases are identified by their doctors. Sleep deficit has become one for the most pervasive health problems facing the United States. Insomnia, both primary and secondary, can result in case accidents and industrial mishaps. It reduces motor, mental, and cognitive functions affecting performance at home, work, and school.

Stages of Sleep

Usually sleepers pass through five stages: 1, 2, 3, 4 and REM (rapid eye movement) sleep. These stages progress cyclically from 1 through REM then begin again with stage 1. A complete sleep cycle takes an average of 90 to 110 minutes. The first sleep cycles each night have relatively short REM sleeps and long periods of deep sleep but later in the night, REM periods lengthen and deep sleep time decreases.

Stage 1 is light sleep where you drift in and out of sleep and can be awakened easily. In this stage, the eyes move slowly and muscle activity slows. During this stage, many people experience sudden muscle contractions preceded by a sensation of falling.

In stage 2, eye movement stops and brain waves become slower with only an occasional burst of rapid brain waves. When a person enters stage 3, extremely slow brain waves called delta waves are interspersed with smaller, faster waves. In stage 4, the brain produces delta waves almost exclusively. Stages 3 and 4 are referred to as deep sleep, and it is very difficult to wake someone from them. In deep sleep, there is no eye movement or muscle activity. This is when some children experience bedwetting, sleepwalking or night terrors.

In the REM period, breathing becomes more rapid, irregular and shallow, eyes jerk rapidly and limb muscles are temporarily paralyzed. Brain waves during this stage increase to levels experienced when a person is awake. Also, heart rate increases, blood pressure rises, males develop erections and the body loses some of the ability to regulate its temperature. This is the time when most dreams occur, and, if awoken during REM sleep, a person can remember the dreams. Most people experience three to five intervals of REM sleep each night.

Infants spend almost 50% of their time in REM sleep. Adults spend nearly half of sleep time in stage 2, about 20% in REM and the other 30% is divided between the other three stages. Older adults spend progressively less time in REM sleep.

As sleep research is still a relatively young field, scientists did not discover REM sleep until 1953 when new machines were developed to monitor brain activity. Before this discovery it was believed that most brain activity ceased during sleep. Since then, scientists have also disproved the idea that deprivation of REM sleep can lead to insanity and have found that lack of REM sleep can alleviate clinical depression although they do not know why. Recent theories link REM sleep to learning and memory.

Organ systems linked to the sleep cycle

Endocrine system

Most hormone secretion is controlled by the circadian clock or in response to physical events. Sleep is one of the events that modify the timing of secretion for certain hormones. Many hormones are secreted into the blood during sleep. For example, scientists believe that the release of growth hormone is related in part to repair processes that occur during sleep. Follicle stimulating hormone and luteinizing hormone, which are involved in maturational and reproductive processes, are among the hormones released during sleep. In fact, the sleep-dependent release of luteinizing hormone is thought to be the event that initiates puberty. Other hormones, such as thyroid-stimulating hormone, are released prior to sleep.

Both sleep and circadian effects interact to produce the overall rhythmic pattern of the pituitary and pituitary-dependent hormones. Some of the 24-h hormonal rhythms depend on the circadian clock (ACTH, cortisol and melatonin), or are sleep related (prolactin and TSH). GH secretion is influenced by the first slow wave sleep (SWS) episode at the beginning of the night. Pulses of prolactin and GH are positively linked to increases in delta wave activity, i.e. deepest phases of sleep, occurring primarily during the first third of the night. Pulses of TSH and cortisol are related to superficial phases of sleep. As a result of the consolidation of the sleep period, the wake-sleep transition is associated with physiological changes with the endocrine system being part of the adaptive mechanism to reduce physical activity during sleep.

Renal system

Kidney filtration, plasma flow, and the excretion of sodium, chloride, potassium, and calcium all are reduced during sleep. These changes cause urine to be more concentrated during sleep.

There is also sleep-related increase in plasma aldosterone levels; an increase in prolactin secretion. There is increased parathyroid hormone release during sleep, which may affect calcium excretion. In general, the following are reduced during sleep: glomerular filtration rate, renal plasma flow, filtration fraction, and the excretion of sodium, chloride, potassium, and calcium. Smaller quantities of more concentrated urine are excreted during NREM sleep than during wakefulness; during REM sleep urine excretion is reduced and concentrated to a greater extent than during NREM sleep.

Alimentary activity

In a person with normal digestive function, gastric acid secretion is reduced during sleep. In those with an active ulcer, gastric acid secretion is actually increased and swallowing occurs less frequently.

Sleep Patterns

How sleep patterns change over lifetime

Along with genetics and circadian rhythms, one important factor that helps determine the amount of sleep a person needs nightly is stage of life.

Newborns sleep an average of 16 to 18 hours a day. By the age of one, children usually sleep 13 to 14 hours with that number decreasing until they reach adolescence.

Teenagers generally require at least eight and a half hours of sleep a day. Some researchers go so far as to say that teens need more than nine and a half hours each day. With the pressures of academics, athletics, dating, etc., few teens are getting enough sleep these days. Also, teens’ internal biological clocks tend to keep them awake later in the evening and let them sleep later in the morning than adults. Some high schools have found that ringing the first bell an hour or so later has helped the performance of students.

Most adults need around eight hours of sleep to function well. Although many people claim they require less, only 10% require significantly more or less sleep.

For adult women, pregnancy and menopause can cause significant changes in sleep patterns. In the first three months of pregnancy, mothers-to-be often require significantly more sleep than usual. Scientists are just beginning to research how menopause effects sleep. They have found that menopause tends to cause a decline in the quality of sleep and can lead to insomnia, snoring and sleep apnea. These changes may be due to changes in hormonal function and the psychological factors associated with menopause or may be caused by the aging process and weight gain associated with menopause.

Contrary to the popular belief, the need for sleep does not decline with old age. While the elderly do find that their slumber becomes more fitful, they continue to need about the same amount of sleep that they needed in early adulthood. For this reason, older people tend to nap often during the day. Experts say that the number of nocturnal awakenings can start to increase as early as the age of 40.

Cornell University researchers did a longitudinal study – they followed people for years – and found – no surprise – that people slept shorter times as they got older. Daytime napping, however, was about the same as people aged. The scientists also calculated that both the homeostatic process and the circadian process declined over time and that the homeostatic process started to decline before the circadian one did.

How Much Sleep Does a Person Need?

The amount of sleep each person needs depends on many factors, including age, health, recent physical exertion, and mental activity.

Infants sleep about 16 hours a day, while teenagers need about 9 hours on average. For most adults, 7 to 8 hours a night appears to be the best amount of sleep, although some people may need as few as 5 hours or as many as 10 hours of sleep each day. There is quite a bit of variation.

Don’t assume you are at one end of the spectrum unless you have paid close attention to your body. If you are drowsy during the day, even during boring periods, you haven’t had enough sleep the previous night. Most people experience a dip in early afternoon – siesta time.

Sleep deprivation – even one or two nights – can vastly affect your need for sleep. Unlike many things in life, sleep time is not something that is routinely changed. You can’t get used to a lower amount of sleep just because it fits your schedule. If you try to, it will affect your judgment and reaction time, even if you are not consciously aware of it.

If you routinely fall asleep within 5 minutes of lying down, you probably have severe sleep deprivation or a sleep disorder. Microsleeps, or very brief episodes of sleep in an otherwise awake person, are another mark of sleep deprivation. In many cases, people are not aware that they are experiencing microsleeps. The widespread practice of "burning the candle at both ends" in western industrialized societies has created so much sleep deprivation that what is really abnormal sleepiness is now almost the norm.

Sometimes you’ll hear that you need less sleep as you get older. But that is incorrect. Seniors often sleep less than young adults and children because they have insomnia. Also, deep sleep stages (stages 3 and 4) in many elderly people declines to a lower percentage of total sleep time, and may even stop completely. People often feel the decline in stages 3 and 4 sleep represents a decline in sleep quality, and they would prefer that it not happen.

Is this newfound insomnia normal? It’s hard to say for any individual person. Some experts consider insomnia a normal part of aging, or it may result from medical problems that are common in elderly people and from the medications and other treatments for those problems. A recent Dutch study found that older people who slept a lot had higher cholesterol levels and lower good high-density cholesterol than those who slept less.