At the beginning of this century medical scientists made a surprising discovery: that we are built not just of flesh and blood but also of time. They were able to demonstrate that we all have an internal "body clock" which regulates the rise and fall of our body energies, making us different from one day to the next. The idea of an internal "body clock" should not be surprising, since the lives of most living things are dominated by the 24-hour night-and-day cycle. The most obvious feature of this cycle is the way we feel tired and fall asleep at night and become awake during the day. If the 24-hour rhythm is interrupted, most people experience unpleasant side effects. As well as the daily rhythm of sleeping and waking we also have other rhythms which last longer that one day and which influence wide areas of our lives. Most of us would agree that we feel good on some days and not so good on others. Scientists have identified the following three biorhythmic cycles: physical, emotional and intellectual. Each cycle lasts approximately 28 days and each is divided into a high energy period and a low energy period of equal length. During the low energy period we are less resistant to illness and tire more easily. The low period puts energy into our "batteries" for the next high period. During the high energy period of a physical biorhythm we are more resistant to illness, better coordinated and more energetic. The "critical" or weakest time is the time of changeover from the high energy period to the low energy period, or vice versa. This "critical" time usually lasts a day. On the critical day of a physical biorhythm, there is a greater chance of accident and illness. Human experience is always individual and we each have our own biorhythmic experiences. Some people experience such enormous physical turbulence on their "physically critical" days that they have to go to bed.