There are two biological clocks that govern our sleep-wake rhythm, the circadian rhythm and the ultradian rhythm.
(1) Circadian rhythm
This is the familiar 24 hour rhythm. Chronobiologists have demonstrated that if we lock ourselves underground, away from all time cues and in free running conditions, our natural endogenous biological clock is about 25 hours. Hence we wake and sleep one additional hour later with each subsequent day. Hence if we normally feel sleepy at 10 p.m. at night, after three days we will be feeling sleepy at 1 a.m., which is three hours later. If we still go to bed at 10 p.m. as before, we will be twisting and turning in bed until 1 a.m. For those who suffer from insomnia, it is most important to reset the biological clock to the 24 hour clock. The way to do this is to wake up at the same time every morning. Chronobiologists call this exercise entrainment of the biological clock to a time cue called Zeitgeber (or synchronizer). In this case the Zeitgeber is the alarm clock waking us up at the same time each morning, even on Sundays and public holidays.
(2) Ultradian rhythm
There is a brief period of low arousal about every 90 minutes, and it is during this period, me 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.
People who do not work or take any exercise during the day will have too much time on their hands. They will tend to have frequent naps during the day and will not need as much sleep at night. For them there is not much difference between the activities during the day and those at night. These people do not accept the fact that they have already had too much sleep in the day and do not need much more sleep at night. They believe they are suffering from insomnia. A good day is normally a busy active day, so that in the evening you feel tired, as if you have achieved a lot and have earned a good night’s rest. Regular exercise will keep your body healthy, keep your muscles strong, your heart and circulation in good shape, and also let off any tension that has accumulated in your body. It has been shown that exercise increases the amount of NREM sleep. However, if strenuous exercise is undertaken too close to sleep time, it may delay sleep onset. Exercise should be finished at least three hours before bedtime. Exercise and activities like aerobics, jogging, tennis, swimming, golf, gardening, and fishing are all very good for health. After all, sleeping well is a sign of good health, and good health starts with good hygiene.
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