The Circadian Rhythm

It is impacted by three main factors: 

• Light: probably the most significant pace setter of the circadian rhythm. Staring into a bright light for 30 minutes or so can often reset your circadian rhythm regardless of what time of day it is.
• The time of day, your daily schedule, and the order in which you perform tasks can all impact your sleep-wake cycle.
• Melatonin: this is the hormone that causes drowsiness and controls body temperature. It increases after dark and decreases before dawn.