A group of volunteers was placed in a windowless room, isolated from all external clues as to the time of day or the day of the week. The importance of circadian rhythm was first established basement of a Munich hospital.

Many of your body's vital signs are governed by circadian rhythm: Neurological and endocrine functions, for example, follow a twenty-four-hour cycle, as do temperature fluctuations, hormone and enzyme production, electrolyte excretion, and sleep/wakeful cycle. Scientists call it circadian rhythm, the pattern of biological cycles that recurs at approximately twenty-four-hour intervals. In terms of these cycles' influence on our everyday lives, there's one particular well-documented phenomenon that deserves our close attention.

This demonstrates just how deeply these basic cycles are ingrained in every aspect of nature. For example, studies have shown that certain plants, sensitive to the cycle of day and night, can be placed in a dark room for days at a time and still continue to open and close their leaves according to the cycle of day and night, though they have no direct exposure to sunlight. These cycles are found throughout the plant and animal kingdoms and have even been observed in isolated cells and unicellular organisms. In other words, these external and internal rhythms are just two expressions of the same carefully ordered natural cycles. In fact, it's not an exaggeration to say that the internal biological rhythm that each of us perceives as his or her own is actually an expression of the external beat of nature.

In order to enjoy perfect sleep it is essential to understand the extent to which the cadence of our internal experience is influenced by the larger rhythm surrounding us, just as a dancer will be drawn to move in time with the beat of an orchestra. There is also important evidence that illnesses such as clinical depression are dramatically influenced by seasonal changes and even by different times of day. For instance, there is evidence that the point in her monthly cycle at which a breast-cancer patient undergoes surgery is a tremendously important factor in determining the outcome of her treatment.

All this is on a grand scale, of course, and modern science has only recently begun to recognize the full extent to which these cosmic rhythms have a counterpart within the physiology of every human being. Tidal rhythms express gravitational effects of the sun and the moon. The moon takes twenty-eight to twenty-nine days to revolve around Earth. It takes 365% days to revolve around the sun.