Historically, sleep was thought to be a temporary death. Sleepers would have no conscious control or volition. This influenced scientific thought for a long while, even up into the beginning of the twentieth century. Many researchers and scientists believed that sleep was a rest for the mind when it went into was completely cut off from the rest of the body. Today though, we know many of these thoughts to be untrue. During sleep, the brain is not cut off from the body; in fact, it very strictly controls the body during resting hours. In addition, we know that the brain is not merely in torpor during sleep, but actually undergoes many biological processes.
The path followed by sleepers has been well documented by twentieth century researchers. The journey of sleep starts out with the soon to be sleeper in a relaxed state of alpha wave production. Contrary to myth, the sleeper does not slowly drift into sleep. Rather, there is a rapid changeover to somnolence over the course of about a minute. During this changeover, the alpha waves of a relaxed state change over to theta waves. This signifies stage one sleep, the lightest of four distinct stages. During stage one sleep, the individual may experience broken or fragmented hallucinations that lack form. Also during this time, brain waves continue to slow in frequency and muscles relax throughout the sleeper’s body. After a point, stage two sleep is achieved. During stage two sleep, theta brain waves are still the dominant pattern in a sleepers brain, however, these theta waves are interrupted by spindles and K complexes. Spindles are brief periods of beta wave activity that show up during stage two sleep. K complexes are high amplitude, high frequency waves that show up individually also in stage two sleep. Approximately twenty minutes after falling asleep, the sleeper moves into stage three sleep. This state is categorized by a switch into low frequency delta waves, relaxed muscles, slowed cardiac and pulmonary rates, lower blood pressure, and a slowing of digestion. Later, the sleeper shifts over to stage four sleep. This is the deepest stage of sleep, and it is difficult to rouse an individual from this state. After this stage, though, the sleeper begins to reverse the process, going back to stage three and then to stage two sleep. Instead of going back to stage one though, a new phenomenon occurs. Brainwaves that had been in the slow delta frequency, switch over into the beta range of normal conscious thought. At the same time, all voluntary muscles relax while the eyes start to move rapidly under the eyelids. This cycle of sleep, known as REM (Rapid Eye Movement) sleep, is the time when dreams occur. It lasts for three to ten minutes. The process described above describes the first sleep cycle. Each cycle lasts for about ninety minutes. Stage one sleep occurs only in the first and last sleep cycles: right after falling asleep and right before waking up. The amount of time spent in stages three and four sleep also decrease every cycle. The brain goes through lighter and lighter sleep as the night progresses. The time spent in these deeper sleeps is replaced with REM sleep so that in the cycle before waking, an individual may experience up to forty-five minutes of dreaming.
Sleep is strictly controlled by the brain. More specifically, the brainstem and hypothalamus. The brainstem has two specialized areas that help control sleep: the raphe nuclei and the locus coeruleus. These areas use the neurotransmitters, noradrenaline, serotonin, and acetylcholine to control sleep. These areas are in turn controlled by our circadian rhythms. Circadian rhythms are our internal clocks that tell the body when to undergo different processes. Using a mixture of hormones and nervous system activity, the circadian clock keeps an almost perfect twenty-five hour day. Since a day on Earth is only twenty-four hours though, the circadian clock requires constant updates to keep it running at the proper time. These updates are received from the external environment. In humans, the suprachiasmatic nuclei, found next to the thalamus, receives direct sensory information from the eyes to help regulate the internal clock.
Sleep is required for well being. People who are deprived of sleep will begin to fall apart physically and mentally. Studies in rats have shown what happens to organisms that go without sleep. The rats would become feverish and, even though their appetite increased, they would lose weight. In addition to these symptoms, the rats would cease to groom themselves, and also developed lesions on their skins. The rats body temperatures would begin to drop as well, and they would eventually die. While, for obvious reasons, this experiment can’t be repeated with humans, it stands to reason that a human deprived of sleep long enough would show many of the same symptoms and maladies of sleep deprivation. One reason that these symptoms may occur is that several hormones are secreted during sleep. These hormones tell cells to repair themselves as well as promoting healing on a tissue level. Scientists believe that the secretions occur during the third and fourth stages of sleep when delta waves are produced. People deprived of delta sleep will begin to develop generalized muscle aches over a period of several weeks to months.
With the discovery of REM sleep, research opened up on dreams and dreaming. Scientists have found that dreams are carried out by almost all of the brain. Activity begins in the lower, more primal regions of the brain and, from there, spreads to the higher more advanced parts of the brain like the neocortex. While dreaming, rapid frequency beta waves are produced, these waves are indicative of concentration and mental activity. During dreams, activated parts of the brain, not knowing that they are asleep, send messages to muscles telling them to move. However, knowing that the brain is asleep, the brainstem sends out messages that cancel the orders sent by the motor controllers in the brain. If this were not the case, dreamers would act out the dream scenes with possibly horrible results. The brainstem allows many functions to be affected by dreaming however. Muscles controlling the eyes and ears are allowed to flex (internally in the case of the ears) trying to see or hear what is being dreamt about. At the same time, heart and respiratory rates increase. This corresponds with an increase in blood flow to the brain to support its flurry of activity.
Many theories exist that attempt to explain why people dream. One researcher suggests that dreaming is exercise for the brain. In the young, it would help to establish neural connections, and in the mature would make sure that everything is in working order. Another theory suggests that dreams are used to program instinctual behaviors. Dreams teach the brain to do things that at first are only available to us in our genetic code. Yet another theory tells us that dreaming is a way to reinforce our memory. Dreams would reactivate the circuits in our brain where memory is stored, thus strengthening them. There is evidence to help hold up this theory in that those deprived of REM sleep often have trouble learning and remembering. Sigmund Freud, a famous psychologist, theorized that dreams were the brains way of showing repressed impulses and desires to the conscious mind. However, since the conscious mind doesn’t want to see much of what is being shown, the dreams come in code. Instead of being forthright, the images in dreams come in symbols and codes that may be interpreted by those who know what they mean. This may be the reason that many people from many different cultures and areas report having dreams with similar themes. These themes include falling, flying, and being chased, as well as the presence of certain animals, fire, and water.
During sleep, an individual may experience scary or terrifying experiences. These tend to fall in to one of two categories: nightmares and night terrors. Nightmares occur during REM sleep. They occur as normal dreams that in some way scare the dreamer. Depending on how horrifying the dream is, the sleeper may stay asleep and continue to experience the dream, or wake in attempt to escape it. In the latter case, nightmares are often remembered by the now awake individual. Night terrors though are a completely different story. Affecting mostly young children, night terrors occur during delta sleep. The child will suddenly wake screaming or crying with very little or no memory of what occurred other than the fact that it was terrifying. He or she may continue to scream or cry for several minutes before calming down. Often the child will then return to sleep and have mostly or entirely have forgotten about the experience by morning. The adult versions of night terrors are called incubus. During stage four sleep, a sufferer of this phenomenon will suddenly wake up. This rousing is accompanied by a feeling of weight on the chest as well as an indescribable feeling of anxiety. The sufferer will not have any memory of any dreams though that caused this state.