Melatonin
Melatonin is a natural substance produced in the body that regulates circadian (night-to-day) cycles and sleep--wake cycles.
Producing melatonin in the brain is stimulated by the dark and decreases with light. Darkness causes the body to build more melatonin, which signals it to sleep, while light inhibits melatonin production and sends the body to be awake. Screen lights may also affect melatonin secretion, which is why it is most beneficial to diminish the use of electronics that emit blue light prior to bedtime.