Human and animals all have an internal “clock” that regulates the ebb and flow of consciousness. The most obvious change in consciousness that you experience daily is the cycle from sleeping to waking. In addition, you're probably also very familiar with the daily change in your levels of mental alertness. For most people, peak mental alertness occurs in the morning around 10 A.M. and again in the evening around 8 P.M.

These clocks are known as circadian rhythms and represent the psychological and biological fluctuations that occur on a roughly twenty-four-hour timetable. How exactly does the human body “keep time”? A tiny cluster of approximately 20,000 neurons in the hypothalamus known as the suprachiasmatic nucleus (SCN) serves as the master control panel for this daily cycle of consciousness. Researchers aren't entirely sure how this process works, but they do know that environmental cues are important.

Sunlight is perhaps the best example of an environmental stimulus that helps regulate the circadian rhythms. When levels of sunlight start to decrease at the end of the day, the visual system transmits this information to the SCN, triggering a message to the pineal gland to increase production of a hormone known as melatonin. This hormone causes the body to slow down activity levels and leads to increased sleepiness.