The circadian rhythm is typically a 24 hour internal clock that runs automatically. It tells your brain when to cycle between sleepiness and alertness. This is also known as a sleep-wake cycle. Part of the brain’s hypothalamus controls the circadian rhythm which works with the pituitary and eyes, among other things. In typical people, the circadian rhythm is highly influenced by light signals. For example, a sunset (dimming lights) triggers the body to make melatonin, which typically prepares people for sleep. The circadian rhythm is incredibly complex, not fully understood, and multi-systemic.