Melatonin

Melatonin is a hormone which is secreted by the pineal gland in the brain, and is used to regulate sleep. Melatonin accumulates through the day and is cleared from the body during sleep. Higher levels of accumulated melatonin in the body will create a desire to sleep in the animal.

Melatonin secretion and accumulation cycles underpin the circadian rhythm found in all animals and even some plants.