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.

Stages of Sleep

Each night's sleep is divided into two major alternating phases:

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: