Early scientists and philosophers saw sleep as a passive condition where the brain is isolated from the other parts of the body. Alcmaeon claimed that sleep was caused by the blood receding from the blood vessels in the skin to the interior parts of the body. Aristotle suggested that while food is being digested, vapors rise from the stomach because of their higher temperature and collect in the head. As the brain cools, the vapors condense, flow downward and then cool the heart which causes sleep. Still others claimed that toxins that poisoned the brain caused sleep (Lavie, 1993/1996). With the discovery of brain waves and later the discovery of electroencephalogram, the way sleep was studied changed forever. Electroencephalogram or EEG made it possible for sleep researcher to record the electrical activity of the brain during sleep.
EEG recordings show that we go through five stages of sleep each with its characteristic brain-wave activity. Stage 1 is the transition stage from wakefulness to sleep and is identified with theta waves and last between 1 to 7 minutes. In stage 2 EEG recordings show fast-frequency burst of activity called sleep spindles. In stages 2 through 4 muscle tension, heart rate, respiration, and temperature gradually decline, and it becomes more difficult to be awakened. 30 minutes after falling a sleep we pass through stage 3 and enter stage 4. In this stage EEG recordings show delta waves and it is the deepest stage of sleep. There is a marked secretion of growth hormone in stage 4. Sleep researchers determine what sleep stage a person is in by the ratio between the number of sleep spindles and the number of delta waves (Munglani and Jones, 1992). After this stage we go back to stage 2 and then enter REM sleep or rapid eye movement sleep. Here EEG tracings look exactly like the beta waves that are observed when we are completely awake. In fact, brain-imaging studies show that the neurons in the cerebral cortex become much more active during REM sleep (Munglani and Jones, 1992). REM sleep makes up 20% of our sleep time and during this stage we experience vivid dreams. We go through this sleep cycle 5-6 times during 8 hours of sleep (Plotnik, 1993).
Michel Jouvet has shown that changes in EEG activity during sleep is due to alternating activity of the raphe nuclei and the locus coeruleus. According to Jouvet the onset of sleep is due to increased activity of the raphe nuclei which secrete serotonin when active. The locus coeruleus which secretes norepinephrine when active, increases REM sleep. The activity of these two sites alternate, which could account for the fluctuation between deep and light sleep throughout the night. Since Jouvet's research, several neurotransmitters have been identified that are involved in the control of REM sleep. Acetylcholine is involved in initiating REM sleep and serotonin and norepinephrine are involved in inhibiting REM sleep (Carlson, 1991). The reticular formation in the hindbrain helps regulate REM sleep and according to Kalat (1995), when the reticular formation receives electrical stimulation, a sleeping person wakes up. It is known that body temperature controlled by the hypothalamus plays an important role in regulating the sleep-wake cycle. So a combination of physiological factors, brain areas, and neurotransmitters seems to be responsible for controlling falling asleep and waking up (Plotnik, 1993).
There are three important factors that determine when we fall asleep:
The above factors explain what determines when we will fall asleep. But the question of why we sleep still isn't answered. There are two different theories of why we sleep:
According to this theory activities during the day deplete key factors in our brain or our body that are replenished and repaired by sleep. This theory makes sense since during stage 4 sleep there is a marked secretion of growth hormone, controlling many aspects of metabolism, physical growth, and brain development. The finding that after having been sleep-deprived for a long period, people tend to sleep longer when they have the opportunity (Moorcroft, 1993) also supports the repair theory.
This theory states that sleep evolved because it prevented early humans and animals from wasting energy and exposing themselves to dangers of predators. It was necessary for their survival.
REM Sleep Deprivation People who are deprived of sleep for one or more days show almost no physical or behavioral changes. However people who are deprived of REM sleep for one or more nights spend more time in REM sleep the next night. This is referred to as REM rebound. According to Vogel REM Sleep deprivation can be beneficial in depressives. He argues that depression results from excessive neural disinhibition during REM sleep. REM dissipates too much accumulated neural excitability, so if the depressives are deprived of REM sleep discharge of neural excitability is prevented, making depressed people more sensitive to their drive states. They would then engage in behaviors that produce rewards, that are "healthy", causing them to experience positive affect (Franken, 1988). On the other hand, Ford and Kamerow (1989) found that people who complain from insomnia for a period of one year are twenty times more likely to develop a serious depression than those who do not suffer from insomnia.
Insects, fish, amphibians, and reptiles all meet the behavioral criteria of sleep which are behavioral quiescence, a stereotypic species-specific posture, elevated arousal threshold, and rapid change in state after intense stimulation (Lavie, 1993/1996). Some animals have really interesting sleeping styles. Some birds sleep for brief periods with one eye closed and for that short moment it is suggested that one hemisphere of the brain shows waves that indicate sleeping, and the other shows signs of wakefulness. Elephants sleep for 3 to 6 hours, of which two hours are spent standing. The dolphin sleeps with only half of its brain while the other half remains alert. The two hemispheres alternate every one to three hours during sleep. Dolphins kept in aquariums usually swim in circles, in the same direction during sleep. There is no solid evidence of whether animals dream which brings us to the dream world of human beings (Lavie, 1993/1996).
Whether we'd like to admit it or not, whether we are able to remember them or not, we all dream. People awakened from REM periods in sleep experiments report they've been dreaming 80%- 100% of the time. REM dreams are considered to be more perceptual and emotional as opposed to NREM (non-rapid eye-movement)dreams. Content of NREM dreams is often a recreation of some psychologically important event. According to Freud REM dreams are like primary-process thinking which is often unrealistic and emotional, and NREM dreams are like secondary-process thinking which is more realistic (Franken, 1988).
Freud's Theory. According to Freud, we dream to satisfy unconscious desires or wishes, especially those involving sex and aggression. If we were to fulfill these wishes during day time it would create too much anxiety. Freud stated that the wishes are represented with symbols since they would otherwise be anxiety producing. Based on this theory a therapist must interprete these symbols to help clients discover unconscious desires.
Activation Theory states that the hindbrain transmits chaotic patterns of signals to the cerebral cortex, and then higher-level cognitive processes in the cerebral cortex try to integrate these signals into a dream plot (Hobson and Stickgold, 1995).
Dreams can also be viewed as extensions of waking life, which include thoughts and concerns especially emotional ones. Then, in a sense dreams provide clues to the person's problems, concerns, and emotions (Plotnik, 1993).
Dreaming sounds like it is the best part of sleeping. There are still a lot of unanswered questions about the reasons for sleep and dreams, their exact functions, and how physiological processes are involved. But we know a lot more than we did at the beginning of the century. Goodnight. Sleep tight.
This site provides information about people with sleep disorders and is supported by an educational grant from Searle. It is linked to The Sleep Well, which is dedicated to provide all kinds of information on sleep disorders as well as centers for treatment.
This site provides information about dreams and symbols and is authored by Richard J. Corelli, M.D.
This sites provides comprehensive information on sleep, beginning with the sleep cycle and continuing with sleep disorders and symptoms associated with them. It is created by students at the University of Medicine and Dentistry of New Jersey.
This site contains information about how lucid dreams can be induced using a device called the Nova Dreamer.
This is a monthly online publication of sleep disorders with useful information on sleep research and sleep labs. It also provides links to other Web sites on sleep.