Restorative theories of sleep describe sleep as a dynamic time of healing and growth for organisms. For example, during stages 3 and 4, or slow-wave sleep, growth hormone levels increase, and changes in immune function occur. The myriad illnesses associated with sleep deprivation testify to its restorative function.
According to the ontogenetic hypothesis of REM sleep, the activity occurring during neonatal REM sleep (or active sleep) seems to be particularly important to the developing organism (Marks et al., 1995). Studies investigating the effects of deprivation of active sleep have shown that deprivation early in life can result in behavioral problems, permanent sleep disruption, decreased brain mass (Mirmiran et al. 1983), and an abnormal amount of neuronal cell death (Morrissey, Duntley & Anch, 2004). Given sleep's heterogeneous nature, however, no single theory predominates, as it is difficult to describe one single "function" of sleep.
One process known to be highly dependent on sleep is memory. REM sleep appears to help with the consolidation of spatial and procedural memory, while slow-wave sleep helps with the consolidation of declarative memories. When experimental subjects are given academic material to learn, especially if it involves organized, systematic thought, their retention is markedly increased after a night's sleep. Mere rote memorization is retained similarly well with or without an intervening period of sleep.
Non-REM sleep is an anabolic state marked by physiological processes of growth and rejuvenation of the organism's immune, nervous, muscular, and skeletal systems. Sleep also restores neurons and increases production of brain proteins and certain hormones. Wakefulness may perhaps be viewed as a cyclical, temporary, hyperactive catabolic state during which the organism acquires nourishment and procreates. Also, during sleep, an organism is vulnerable; when awake it may percieve and avoid threats. Asking the question "Why do we awaken?" instead of "Why do we sleep?" yields a different perspective toward understanding how sleep and its stages contribute to a healthy organism.
One view, "Preservation and Protection", is that sleep serves an adaptive function. It protects the individual during that portion of the 24-hour day in which being awake, and hence roaming around, would place the individual at greatest risk. Organisms don't require 24 hours to feed themselves and meet other necessities. From this perspective of adaptation, organisms are safer by staying out of harm's way where potentially they could be prey to other stronger organisms. They sleep at times that maximizes their safety, given their physical capacities and their habitats. (Allison & Cicchetti, 1976; Webb, 1982).
Another view is that the function of sleep is for memory processing. This theory argues that saving memory directly into the long-term memory is a slow and error prone process, and, thus, proposes that the memory formed during waking time is not saved directly into the long-term memory; instead it is saved into a temporary memory store first. The function of sleep is to process, encode and transfer the data from the temporary memory store to the long-term memory store. (Zhang, 2004).
Recent research suggests that sleep patterns vary significantly across cultures. [1] The most striking differences are between societies that have plentiful sources of artificial light and ones that do not. The primary difference appears to be that pre-light cultures have more broken up sleep patterns. For example, people might go to sleep far more quickly after the sun sets, but would then wake up several times throughout the night, punctuating their sleep with periods of wakefullness, perhaps lasting several hours. The boundaries between sleeping and waking are blurred in these societies. Some observers believe that sleep in these societies is most often split into two main periods, the first characterised primarily by "slow sleep" and the second by REM sleep. This is called segmented sleep, which led to expressions such as "first sleep" "watch" and "second sleep" which appear in literature from all over the world in pre-industrial societies.
Some societies display a fragmented sleep pattern in which people sleep at all times of the day and night for shorter periods. For example, many Mediterranean societies have a siesta, in which people sleep for a period in the afternoon. In many nomadic or hunter-gatherer societies people will sleep off and on throughout the day or night depending on what is happening.
Plentiful artificial light has been available in the industrialised west since at least the mid-nineteenth century, and sleep patterns have changed significantly everywhere that lighting has been introduced. In general people sleep in a more concentrated burst through the night, and sleep much later, although this is not always true.
In some societies people generally sleep with at least one other person, often many, or with animals. In others people rarely sleep with anyone but a most intimate relation such as a spouse. In almost all societies sleeping partners are strongly regulated by social standards. For example, people might only sleep with their immediate family, extended family, spouses, with their children, with children of a certain age, children of specific gender, peers of a certain gender, friends, peers of equal social rank, or with no one at all. Sleep may be an actively social time depending on the sleep groupings, with no constraints on noise or activity.
People sleep in a variety of locations. Some sleep directly on the ground, others on a skin or blanket, others sleep on platforms or beds. Some sleep with blankets, some with pillows, some with simple head rests, some with no head support. These choices are shaped by a variety of factors such as climate, protection from predators, housing type, technology, and the incidence of pests.
common misperception is that everyone needs eight hours of sleep. The amount of sleep needed is different for each person. This amount needed is individually and biologically determined. Some can do with six hours of sleep, others need nine. However, as a general rule, eight hours is recommended. Sleep experts state that you cannot "store" sleep by sleeping more on the weekends in preparation for the normal work week. Just how much sleep deprivation leads to death in humans is unknown.[2]
Another commonly held view is that the amount of sleep one requires decreases as one ages, but this is not necessarily the case. The ability to sleep, rather than the need for sleep, appears to decrease when people get older. [3]
Failure to sleep results in progressively severe psychological and physical distress. In 1965, California teenager Randy Gardner attempted to resist sleep in an uncontrolled "experiment". As his ordeal progressed he fell into a silent stupor, bringing into doubt whether he was actually awake in any practical sense.
A 1999 University of Chicago team led by Eve Van Cauter limited a group of lean young men to four hours of sleep per night for sixteen days. The subjects showed decreased levels of leptin and increased levels of cortisol. The subjects also increased their daily caloric intake by 1,000 calories. The team discovered that the subjects' insulin and blood sugar levels resembled the impaired glucose tolerance of prediabetics, an indication that they were no longer properly processing carbohydrates. Studies have also linked sleep deprivation to an increased incidence of obesity.
At Harvard Medical School, researchers have identified associations between sleep deprivation and illnesses ranging from hypertension and heart attacks to cancer. Poor sleepers generate increased levels of stress hormones and show more inflammatory changes in the walls of their small blood vessels, both of which contribute to elevated blood pressure. Because of their exposure to light at night, night-shift workers produce less melatonin, a hormone which not only promotes sleep but has been shown to have cancer-prevention benefits as well.
Experiments with rats have measured the effects of long-term sleep deprivation. In one experiment, a pair of rats were placed on a circular rotating platform and separated by a wall. Both were instrumented with electroencephalograms. Whenever the "subject" rat began to show signs of sleep, the platform rotated, forcing both rats to either walk in the direction opposite to the rotation or be forced off the platform into shallow water. The "control" rat was allowed to sleep while the "subject" rat was awakened. After approximately three weeks the "subject" rat became unable to regulate body temperature; even if allowed to sleep at this point, it died shortly afterward from septic shock.
In sleep-deprived states less extreme than that suffered by Randy Gardner, humans display irritability, impaired cognitive function, and poor judgment. Experiments on sleep-deprived medical trainees, for example, have shown them less able to interpret EKGs and x-rays than their well rested peers. As late as the early twenty-first century people thought that too little sleep could be negated by "paying back the sleep debt". However, recent studies have shown this to be false[citation needed].