Melatonin's primary role is to signal daytime and nighttime to the brain and body, indicating when it is time to sleep, but it does not generate sleep itself.

“And then melatonin, we think of that as a sleepiness hormone, but it has fairly weak soporific activity, which means it's not a very strong stimulator for sleep, but it's more of a reinforcer that it is in fact dark out. And so it's telling all the clocks within the suprachiasmatic nucleus, "Yeah, it's now nighttime. Do nighttime activities." So it will affect the transcription and the translation activity that's happening.”