The theory of sleep as a biological process is relatively new, dating back to the 1950’s when REM sleep was first discovered (Aserinsky & Kleitman, 1953). Until that time, there were a couple of central misconceptions regarding the sleep-wake cycle. Sleep was commonly thought to be a mechanism through which the body compensated for a daily build-up of hypnotoxins. A person was thought to fall asleep when the level of hypnotoxins in the blood became too high.
The person would then awaken when these toxins were expelled. A second problem with early sleep research was the faulty notion that the sleep-wake cycle was not endogenous. That is, some researchers attributed the sleep-wake cycle to environmental factors that acted upon a person rather than a person’s intrinsic biological rhythm. They thought that factors such as levels of light caused the cycle to occur when in reality this is not the case. The cycle persists even when external cues are eliminated.
The Sleep Cycle:
Normalcy, Insomnia, and its Relationship to Depression
Sleep is a biological need shared by all humans, yet it is only vaguely understood by researchers. We do know that healthy sleep patterns are related to other signs of physical and mental wellness. Conversely, problems sleeping are symptomatic of other health risks. In extreme cases, disordered sleep can be attributed to the cause of other serious disorders. It can lead to disastrous accidents. Therefore, the importance of sleep, and of understanding sleep, is vital.
When Aserinsky and Kleitman (1953) discovered REM sleep, scientific studies of sleep became more common. Still, it was not until the 1980’s that sleep researchers began to focus on the sleep-cycle, its causes, and dreaming.
Today, researchers have adopted a perspective that is similar to the view of early researchers. Although the notion of hypnotoxins has been abandoned, it is still thought that wakefulness over a period of time causes the body to develop a “sleep debt” which can be paid through sleep.
Human Biological Clock
The sleep-wake cycle is a type of circadian rhythm, which is a biological cycle that repeats itself approximately every 24 hours. Physiologists have identified a network of structures and chemicals in the brain which control the sleep-wake rhythm. Collectively, this network serves the function of the human biological clock.
The suprachiasmatic nucleus (SPN) is a small structure in the hypothalamus and is the central pacemaker of the body. Lesions of the area of the brain disrupt circadian rhythms, demonstrating the great importance of this area in the regulation of the sleep-wake cycle (L. Becker, personal communication, March 1, 2004).
When certain retinal receptors are exposed to light, they send information to the SPN. The SPN sends input to the pineal gland, which is responsible for the secretion of a hormone called melatonin. The secretion of melatonin from the pineal gland helps to resynchronize the body’s biological clock. Melatonin is produced only at night; light inhibits its synthesis.
Stages of Sleep Cycle
When the rhythm is properly synchronized, an individual will cycle through several hours of wakefulness followed by a period of sleep. The sleep cycle can be divided into five stages.
The first four stages of sleep are categorized together as non-REM sleep because there are no rapid eye movements during these stages. Non-REM sleep is also characterized by varying degrees of brain activity, which is measured by an electroencephalograph (EEG). Stage one is a brief transitional period of light sleep. During this stage, breathing and heart rate begin to slow. Body temperature decreases and muscles begin to relax.
Theta waves are prominent in the brain during this stage. While the amount of time it takes to fall asleep varies from person to person, most people spend 1-7 minutes in the first stage of sleep. Stage two follows and is characterized by sleep spindles, which are brief bursts of higher frequency waves. EEG studies reveal mixed brain activity during stage two sleep, which lasts approximately 10-25 minutes.
Finally, about 30 minutes after falling asleep, individuals reach slow-wave sleep, which is comprised of stages three and four. Slow-wave sleep receives its name from the high amplitude, low-frequency delta waves that are present during sleep stages three and four. Sleep is deeper in stages three and four than in the first two stages. A person will remain in slow-wave sleep for about 30 minutes before cycling back through the lighter stages of sleep and finally into REM sleep.
REM sleep is somewhat mysterious. During REM sleep, the brain demonstrates high-frequency beta waves that mimic wakefulness. Blood pressure rises and respiration increases. Another characteristic of this stage of sleep is the presence of rapid eye movements, which is how REM sleep got its name. In spite of the apparent burst of activity during this phase, individuals are actually in a deep sleep. Their muscles are very relaxed and they are difficult to awaken. The beta waves and eye movements, therefore, are attributed to the process of dreaming. Indeed, most dreaming does occur during REM sleep, although it is possible to dream in non-REM sleep as well.
Variations in the cycle. Slow wave sleep is most prominent early in the night. Individuals will spend gradually less time in slow wave sleep and more time in REM sleep as they cycle through the sleep stages. The first REM period during the night lasts only a few minutes, while the last one or two periods may last up to an hour.
The sleep-wake cycle varies significantly with age. Young adults spend about 20% of their total sleep time in slow-wave sleep and 20% in REM sleep, suggesting that fluctuations in slow-wave and REM duration even themselves out during the night. However, this is not true of other age groups. Infants spend a great deal more time in REM sleep than adults. In addition, older adults spend dramatically less time in slow-wave sleep than younger adults. There is also a decrease in total time spent sleeping in the elderly, despite an increase in the amount of time spent in bed.
Clearly, the sleep-wake cycle has variations according to individual differences. Stimuli outside the individual also have the potential to disrupt the rhythm. These factors include periods of stress, illness, or mental dysfunction. When the sleep-wake cycle becomes irregular, sleep may become disturbed.