Stages of Sleep
Stages of Sleep(dreams)
One of the components of contemporary scientific sleep research is the classification of sleep stages according to certain physiological indicators. Most commonly, the two measurements used to classify sleep are brain wave patterns (measured with an electroencephalogram, or EEG) and eye movements which, during sleep, periodically exhibit rapid activity, referred to as rapid eye movement (REM).
Just before a subject falls off to sleep, an EEG machine will record a characteristic brain wave pattern referred to as alpha waves, which has a frequency of eight to thirteen hertz (cycles per second). As we pass into sleep, the EEG shows a relatively low voltage and mixed frequency of two to seven hertz. This transitional state is referred to as Stage l sleep. Stage 2 sleep is characterized by EEG patterns referred to as transient sleep K-complexes and spindles. Stages 3 and 4 are characterized by delta waves (having amplitudes of more than seventy-five microvolts for at least half a second). In Stage 3, the EEG consists of 20 percent to 50 percent delta waves; in Stage 4, the EEG consists of more than 50 percent delta waves.
In the normal sleep cycle of a young adult, the sleeper initially progresses from Stage 1 (lasting half a minute to 10 minutes) to Stage 2 (20 to 45 minutes) to Stages 3 and 4 (these together are referred to as deep sleep and may last anywhere from a few minutes to an hour). The sleeper eventually returns to Stage 2 sleep, and then enters a REM state, typically for a few minutes. At this point, the sleeper has completed the first sleep cycle. After the initial REM state, the sleeper goes back to Stage 2 sleep, repeating the entire sequence all over again (with the absence of Stage 1 sleep, which is more of a transitional state than an actual sleep stage). In each succeeding cycle, the duration of REM sleep (in which most dreaming occurs) becomes longer and longer, until by morning REM periods can last as long as an hour.
The length of a sleep cycle also varies according to brain size. (For instance, the length of a sleep cycle for a cat is 30 minutes.) Typically, in an adult man, a sleep cycle lasts about 90 minutes, and healthy adults tend to go through between four and six such cycles every evening. There is much variation in the cycle with respect to age, and one’s sleep tends to become progressively lighter and more broken up with age.