Sleep and Dreaming
The average person averages from seven to eight hours of sleep each day, which figures out to around one third of life spent in this altered state of consciousness. Given that so much of one's time is devoted to it, sleep must serve an extremely important function. Yet the question of why we sleep has turned out to be a surprisingly difficult one to get a clear answer to. Here are some possibilities:
Why We Sleep
Note that these theories are not mutually exclusive -- any or all of them could be a part of the explanation for why we sleep.
- Restoration Theories
These suggest that we sleep in order to repair and restore the body after the day's activities. There are two restoration theories:
- Restoration of the body -- we sleep in order to restore the body. But this can't be the whole story, because restoration of the body could occur just by taking it easy for a few hours. Being asleep isn't really necessary. Also, conjoined twins to not always sleep at the same time. If something in the bloodstream signaled the brain that the body needed restoration, one would expect both twins -- who share a common blood supply -- to be tired and rested together.
- Restoration of the brain -- we sleep in order to give the brain a rest, allowing it to restore supplies of neurotransmitter and synthesize needed protiens. This may be so, but it is worth noting that during sleep the brain is expending about as much "power" in neural impulses as during waking. Thus, on average at least, the neurons of the brain are not really "resting" during sleep.
- Evolutionary Theories
These suggest that we sleep because sleeping helps us to adapt better to conditions, thus improving biological fitness. Two evolutionary theories are"
- Avoidance of danger -- we sleep in order to reduce the chances of being discovered by predators during those times of the 24-hour day when our senses do not work as well (for humans, during night when we can't see as well).
- Reduction of need -- we sleep in order to conserve scarce resources (food, water). Metabolic rate tends to be lower during sleep, so an organism that sleeps a goodly part of the day needs less food, water, etc. to survive. This could be important if these resources are hard to come by.
Stages of Sleep
Each night's sleep is divided into two major alternating phases:
- Slow-wave sleep (NREM Sleep)
This is characterized by relatively high-voltage, low frequency brain waves, which indicate that large populations of neurons in the cerebral cortex are firing together in waves, or in other words, in a synchronized fashion (cortical synchrony). Slow-wave sleep has been divided into four stages, ranging from Stage 1 (light sleep) to Stage 4 (deep sleep). The brain waves are progressively of higher voltage and lower frequency as one moves from waking to Stage 1 through Stage 4.
- Rapid Eye Movement (REM) or "Paradoxical" Sleep
REM sleep has three major characteristics:
Dreaming typically occurs during REM sleep. However, it is possible to have something resembling a dream during slow-wave sleep, and such things as night terrors and sleep-walking occur during slow-wave sleep, not during REM sleep.
- Rapid eye movements -- the eyes dart about under the eyelids, as if the person were watching some action going on in front of him or her. These, however, turn out not to be related to dream content, but are generated in the brainstem at the level of the pons.
- Fast, low-voltage brain waves -- the brain waves resemble those generated in the cortex when the person is awake. Because from the brain waves the person seems to be awake, yet is really in a deep sleep, REM sleep is also called "paradoxical" sleep.
- REM paralysis -- the large, antigravity muscles of the body are in a state of paralysis; they cannot be voluntarily moved except for a few small twitches.
Pattern of a Typical Night's Sleep
During the first hour of sleep, we gradually descend through the stages of slow-wave sleep, from Stage 1 to Stage 4, and remain in Stage 4 for perhaps a half hour. We then reverse direction and ascend toward Stage 1. However, instead of entering Stage 1, we enter the first REM phase. During the night we alternate between slow-wave and REM sleep, entering the REM phase about every 90 minutes or so. As the night goes on, we spend less and less time in the deeper stages of slow-wave sleep and more time in REM sleep.
Why We Dream
Nobody knows for sure why we dream. Sigmand Freud suggested that dreams allow us to express forbidden thoughts in a disguised form, a kind of safety valve mechanism. However, there is essentially no evidence for this. Another suggestion is that dreams represent the fragmentary activation of memories of events and thoughts while the memory system organizes the day's experiences so that they can be accessed more efficiently. However, sleep deprivation does not seem to disrupt a person's ability to form and retrieve memories. Yet another suggestion is that dreams are created from one's experiences, fears, and so because during sleep the cortex is deprived of much of its normal inputs. Whatever the reason, dream content typically is about things, people, and concerns that occupy our minds when we are conscious.
Disorders of Sleep
Sleep can be disordered in many ways; here is a brief list of a few of the more common or interesting sleep disorders:
- Insomnia -- difficulty in getting to sleep or staying asleep. Anything that arouses the brain, such as worries, aches and pains, and so on, tends to make it more difficult to get to sleep and stay asleep. In addition, keeping irregular hours (as in jet lag) can bring on symptoms. A common cause of insomnia is the use of sleeping pills, which in some cases produce an abnormal sleep and induce a rebound insomnia when they wear off.
- Sleep apnea -- breathing stops for periods of up to a minute or even longer. The build-up of carbon dioxide in the bloodstream arouses the person enough to kick-start the breathing process again, at least until the person falls again into a deep sleep. The frequent arousal means that sleep quality is poor and the person may feel tired all the time without knowing why.
- Narcolepsy -- Falling asleep spontaneously, even in the middle of some activity, without wanting to. Other symptoms may include cataplexy (REM-type paralysis while awake, usually triggered by anything that is emotionally arousing) and hypnogogic hallucinations -- dream-like intrusions into waking.