The Time of Your Life

Your body clock also directs manual dexterity, ups and down in reaction time, and the the rise and fall of body temperature and blood pressure over the day. It sparks hunger pangs before your usual meal times, controls how fast medications work, and directs the timing of hundreds of functions that make your body run smoothly.

Your body clock ticks in sync with the earth’s light/dark cycle, and daily exposure to sunlight anchors your inner clock to a 24-hour schedule. Cut off from sunlight, the clock runs on a schedule that is close-to, though not exactly, 24 hours long.

Daily life on earth conforms to our planet’s light/dark cycle. Living things stay most active when they can find food most easily. For humans, that’s the daytime.

The invention of electric lights in the late 19th century made it possible for people to work and engage in other activities any time of day or night. Nevertheless, people still function better in the daytime, and sleep more soundly and longer at night.

Even if they sleep in the daytime, and work at night, people still experience bouts of sleepiness, typically between about 4 AM and 6 AM. On days off, most people—regardless of their work hours—prefer to stay awake in the day, and sleep at night.