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.