Generally free-running (controlled by endogenous pacemakers) and maintains a cycle of about 24-25 hours, even in the absence of external cues - our body releases the hormone melatonin which peaks in the hour of darkness to make us feel sleepy and drops when we wake
When we are awake for a long time, homeostasis tells us to sleep as it lacks energy
The sleep-wake cycle can be influenced by external cues (exogenous zeitgebers) like light/dark, clocks, meals etc.
