SLEEP HYGIENE: BIOLOGICAL CLOCKS
There are two biological clocks that govern our sleep-wake rhythm, the circadian rhythm and the ultradian rhythm.
Ultradian rhythm. This is the 90 minute cycle that goes on round the 24 hour clock. There is some practical significance of this rhythm to those who have sleeping problems.
There is a brief period of low arousal about every 90 minutes, and it is during this period, the 90 minute window, that we feel sleepy and fall asleep quite easily. If for any reason we miss this window, for example, because we are watching television and waiting until a program finishes, we may find it difficult to sleep. We have to wait until the next window, which is 90 minutes later. Hence if you feel sleepy at night, do not wait, go to sleep immediately, or else you may have to wait until the arrival of the next window. It does not follow that if we feel sleepy, say, at 10 p.m. we will feel even more sleepy at 10.30 p.m. In fact, for some people, having passed their 90 minute window at 10 p.m., they will feel wide awake at 10.30 p.m.
Some sleep experts suggest that if you miss the 90 minute window and feel distressed in bed and not sleepy, it may be a good idea to do something relaxing in another room and come back to bed once you feel sleepy again. Remember, the 90 minutes is an average of cycles varying from 60 minutes to 130 minutes.
If you are really very sleepy, you will fall asleep at any time, irrespective of the 90 minute window. This is because, in sleep deprived individuals, the ultradian rhythm disappears.
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