We also carry within us our own biological clocks. The human heart beats 70 times on average each minute. Our moods, alertness, and appetites follow regular patterns, depending on the time of day, the lunar cycle or the season.
Our biological clock seems to be intimately associated with a group of nerve cells in our brain's hypothalamus.
These cells are linked to the retina of our eyes and appear to regulate cycles of hormone secretion, our skin temperature and cycles of rest and wakefulness. The hormone melatonin is thought to play a significant role in controlling our daily (circadian) rhythm.