Melatonin can promote sleep in humans by strengthening the nighttime signal, potentially helping with falling asleep faster, staying asleep, and sleeping more restfully, but it is almost universally useless for insomnia caused by conditioned arousal.

“To to the degree to which your body gets a nighttime signal and that makes you sleepy. Yes, it can promote sleep. Melatonin can promote sleep in humans. For that reason, it can help you fall asleep faster. It can help you stay asleep. It can help you sleep more restfully because it's strengthening that nighttime signal. That's what it does. It's also why it is almost universally useless for insomnia because if you have a conditioned arousal, remember now now everyone knows what conditioned arousal is. If you have a conditioned arousal, your body already knows it's nighttime, still can't sleep. So taking melatonin is almost never going to work to treat an insomnia condition.”