THE 90 MINUTE SLEEP CYCLE
In 1963 Kleitman postulated that the rhythmic recurrence of REM sleep is only a part of a biological rhythm which is continuous in both sleep and wakefulness. He called this the basic rest activity cycle (BRAC). In 1967 Franz Halberg, a scientist working in the USA, named such cycles the ultradian rhythm, which is also known as the 90 minute cycle or the REM/NREM cycle.
The hypothesis concerning the 90 minute cycle is as follows. We know that each sleep cycle consists of REM and NREM stages and that each sleep cycle lasts about 90 minutes. This is believed to be a basic biological rhythm innate in our state of awareness. The 90 minute cycles go right round the 24 hour clock. Every 90 minutes there is a window of a few minutes duration during which a person feels sleepy and can fall asleep. This is why some insomniacs, if they miss the sleep window, may find it hard to fall asleep until the arrival of the next window 90 minutes later. This 90 minute cycle appears to be REM-stage related, and, during the window, other REM-related phenomena may be noted, such as day dreaming, penile erection, or just poor concentration.
Much research was conducted to demonstrate the existence of the ultradian rhythm. Extensive work was carried out on cats and monkeys to chart the activities of these animals in relation to their EEG recordings. It was found that, during the awake state, fluctuations in their activities correspond with the stage in the REM/NREM cycle.
However, the most convincing experiments were carried out by La vie and Scheson in 1981. They tested human subjects in the sleep laboratory. The subjects were instructed to close their eyes and to fall asleep if they could during a 5 minute period of darkness occurring every 15 minutes over 12 hours. It was demonstrated that EEG recordings of stage 1 sleep were evident every 90 minutes but not at other times during the experiments. It was also demonstrated that, when these subjects were sleep-deprived and were very sleepy, their ultradian rhythm disappeared. In other words, when one is very sleepy, one- can fall asleep at any time irrespective of the 90 minute window of the ultradian rhythm. It is also now apparent that this 90 minute cycle is not exactly 90 minutes but can vary from 60 minutes to 130 minutes, with a mean of 90 minutes.
The present controversy over this 90 minute cycle is, when a person falls asleep, how are the cycles relating with each other between the awake state and the sleeping state? Most researchers favour the suggestion that, when a person falls asleep, the first period of NREM sleep or the first sleep cycle appears to reset the 90 minute cycle for the rest of the 24 hours. Also there seems to be a phase reversal after this first NREM sleep. After falling asleep, the brain activity of each REM stage is highly aroused with dream experience. However, during the awake state the 90 minute windows which are REM-related are of low arousal.
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