The sleep/wake cycle is controlled largely by your brain’s biological clock, which also controls other daily rhythms, such as changes in body temperature, blood pressure and the release of hormones.These are also known as circadian rhythms, from the Latin for “around the day.”
A key player in this clockwork is the hormone melatonin, which is produced at night by the pineal gland in the brain and promotes drowsiness. Light, especially blue light from the sun, suppresses this “darkness hormone.” This is one way daylight contributes to alertness, and why darkness at night increases sleepiness.
Habit can also have a powerful effect on the sleep/wake cycle. If you are used to falling asleep at 11 p.m., you will want to go to bed then. Shift workers can get used to sleeping during the day or napping at odd hours, though this is difficult and unsatisfactory for most people and has been linked to health problems.