Our circadian rhythm (or ‘body clock’) is influenced by melatonin, as it acts as a ‘time cue’, anticipating sleep. It is also an important physiological regulator of the sleep-wake cycle. Melatonin is not a sedative, but a sign of darkness or night-time, and is a cue for innate night-time behaviour.In the morning when plasma levels of melatonin decrease, sleep ends and wakefulness begins.elatonin – A signal of darkness and marker for circadian rhythms. Melatonin is a naturally occurring hormone that is key to sleep and the sleep-wake cycle in humans and animals.