Human beings have evolved in a world where there are regular cycles of day and night, and we have thus developed a kind of internal clock that makes us want to sleep at night and be awake during the day. This occurs even when we are, say, underground or in an enclosed factory with no windows where there are no visual cues as to the time of day. The circadian rhythm controls such bodily functions as peristalsis (movement of food through the digestive tract, leading eventually to excretion), blood pressure, melatonin secretion, hormone levels, and alertness versus fatigue. In our modern world, people have tried to adjust to lifestyles that are not exactly commensurate with their circadian rhythms. Electricity and other sources of power allow us to work late at night, drive vehicles, and operate machinery at times when our bodies would rather be asleep. The result is that more accidents involving cars and factory equipment happen at night than during the day. We are also much less efficient at night, even when doing safe activities like balancing the checkbook.