The human animal is unique with regard to the amount of time spent in sleep – at least a third of every day. Lack of sleep has a serious effect on both physical and mental health. Research has shown that sleep occurs in phases with different levels and that dreams occur during shallow REM (rapid eye movement) sleep. There is still much to be found out about the function of sleep and dreaming. While asleep, memories appear to be consolidated; and dreaming seems to be necessary in the processing of emotions.

There is a further aspect of sleep which has yet to be even recognised, let alone explored. The importance of the colon and its friendly inhabitants to human health was commented on previously (see Your secret garden), and the reflex connection between the intake of food and defaecation explained. The products of bacterial digestion in the colon benefit human health in ways which have yet to be fully explored. But what has the colon to do with sleep?

During our usual eight or so hours of sleep, we do not eat or defaecate. By the time we are asleep the contents of the day’s meals have been mostly digested in the small intestine and the indigestible residues have reached the colon. Colonic fermentation then proceeds without interruption and the chemical products have ample time to be absorbed and reach all parts of the body. As in ruminants, such as cattle and sheep, and in many other animals, the day is an active time for eating and night a relaxed time for digestion.

Human sleep is undoubtedly important in mental health, but it may also be essential for physiological health that the fermentation process in the colon is completed and its products fully absorbed.