Your circadian rhythm determines your body’s natural sleep pattern. Circadian rhythm refers to the mental, physical, and behavioral cycles that determine when and how long you sleep and are awake during a 24-hour period. Your natural biology and environmental factors work in tandem to determine when your body creates certain hormones, including melatonin, that will signal your body to sleep or wake up. Daylight is one of the most important environmental signals your body will use to determine when it is appropriate to sleep. People whose careers involve shift work, or who often work overnight, are more susceptible to circadian rhythm disorders that can reduce the amount of sleep they are able to achieve.