Through out the centuries, dating back to Aristotle, humans have been interested in explaining the functions and mechanisms of sleep. Much research has been devoted to answering questions such as why we sleep and why we spend one third of our lives in sleep. However, the explanation behind wakefulness and sleep is still in its infancy. What is certain about wakefulness and sleep is that they both involve complex physiological and psychological mechanisms. Off course, these functions and mechanisms vary in different species, but to define sleep in humans, certain criteria are used:
Consciousness or awareness of the external environment is the line drawn between wakefulness and sleep. Falling asleep and losing consciousness involves a gradual progress of stages. A fully alert and awake individual is in stage 0. Stage 1 is accompanied by being drowsy and drifting in and out of sleep. Then the individual enters stages 2, followed by 3, and eventually 4. After stage 4 he/she will reverse the sequence by returning to stage 3, followed by 2, then 1-REM (rapid eye movement). This cycle takes between 90 to 120 minutes, due to individual differences, then repeats itself. Therefore, in 7 to 8 hours of sleep, this cycle repeats about 5 times (Franken, 1994).
To explain these stages in more detail, brain wave patterns recorded by EEG machines are useful. During wakefulness or at stage 0, alpha and beta activities are experienced in the human brain. Alpha activities consist of medium frequency waves. Beta activities consist of irregular low amplitude waves which are present when the individual is very alert and attentive. As the individual gets drowsy and enters stage 1, brain experiences theta activities. This is the transition stage between wakefulness and sleep. Stage 2 sleep contains irregular theta activities where sleep spindles (short bursts of waves of 12-14 Hz) and K complexes (sudden sharp wave forms) are present. Stage 3 sleep contains high-amplitude delta activities 20 to 50 percent of the time. Stage 4 is very similar to stage 3. It also contains delta activities; however, they are present for more than 50 percent of the time. About 90 minutes after the onset of sleep, when the individual enters REM sleep, EEG patterns become very similar to those obtained during stage 1 sleep.
During REM sleep brain waves indicate theta activities which are very desynchronized (desynchronized means unregulated, e.g. trying to listen to several conversations simultaneously). This stage is also referred to as paradoxical sleep since brain activities during REM are comparable to those during wakefulness. REM lasts about 20 to 30 minutes. In 7 to 8 hours of sleep about 2 REMs are experienced (Carlson, 1991).
Although the entire nervous system becomes very active during REM, movements such as walking and talking are not present, because the muscles become atonic or paralyzed (Franken, 1994). This is due to the function of "locomotor centre" which produces paralysis in voluntary muscles. If this region is destroyed, the animal will move around during REM sleep (Horne, 1988).
In addition to changes in brain wave patterns, brain chemicals also fluctuate during sleep. The two major neurotransmitters involved in sleep are serotonin and norepinephrine. At the onset of sleep serotonin is secreted which increases NREM (non-rapid eye movement, stage 1 - 4 sleep). Secretion of norepinephrine takes place during REM resulting in increase of REM. Fluctuation between stages of sleep are thought to be due to secretion of these two neurotransmitters (Franken, 1994). Notably, a successful treatment of depression is to awaken the patient at the onset of REM sleep. This regulates the imbalance of norepinephrine and serotonin, alleviating depression (Carlson, 1991). However, additional findings about REM deprivation suggest an increase in aggression which lasts after REM deprivation is discontinued (Ellman & Antrobus, 1991).
At this point, it is important to discuss some of the experiments performed on humans and other animals involving sleep deprivation. In a study with human subjects Dinges and Kribbs discovered that performance on short tasks is not impaired when individuals are sleep deprived; however, performance on longer tasks which require sustained attention becomes impaired. In other experiments subjects report perceptual distortions or even hallucinations (Franken, 1994). Rechtschaffen's study on rats that were sleep deprived between 5 to 33 days showed severe effects. During the study, the rats began to look sick and stopped grooming themselves. They became weak and uncoordinated. Some of them died and some had to be sacrificed. On autopsy, enlarged adrenal glands, stomach ulcers, and fluid in lungs were found among these rats; note that these are some of the signs of stress (Carlson, 1991).
After this brief introduction to the mechanism of sleep and wakefulness, what is apparent is the need for all animals to sleep. But the question still remains: "Why do we sleep and wake-up?". To answer this question, first, three factors involved in sleep and wakefulness need to be considered,
Taking the above factors into account, three theories have been developed which attempt to explain the reason behind sleep and wakefulness.
Many theories have also been developed which attempt to explain the function of dreams. However, as much as all of these theories may be true, none fully determines the function of dreams. Could it be that we are trying to resolved the unresolved issues of our lives in dreams? Or perhaps, we are trying to reach an emotional balance since dreams are our means of communication between the unconscious and conscious (Moorcroft, 1993). The answer to the true function of dreams is still ambiguous.
From all the theories developed which attempt to explain why we
sleep and wake-up, a conclusion may be drawn that sleep is not the
motivation for cognitive processes, behavioral adaptation, and
restoration to take place. But survival is the motive behind
sleep. Sleeping is only the behavior which satisfies the
motivation to continue and survive. Perhaps, this is how Charles
Darwin would have approached it.
This is an excellent web site with the most updated information on sleep. It has over 12,000 bibliographic references in addition to a selection of complete articles. Some of the topics explored within this web site include: Sleep-waking cycle, Dreams, Psychological and philosophical aspects, etc.Welcome to the ASDA Home Page.
This the home page of the American Sleep Disorders Association. It provides many useful information on sleep research and sleep medicine. It also provides additional links to other sleep-related web sites.Internet Resources on The Brain, Sleep and Dreams.
This is an excellent web site which provides many resources on the physiology of the brain, states of sleep, dream map, etc. This web site provides additional information on neurological and behavioral disorders as well.Clinical Research Discussion Forum.
This web site's focus is on REM sleep behavior disorders. It also provides answers to the most frequently asked questions on sleep and sleep disorders.