How does your serotonin levels affect when you fall asleep?

Sleep chemical: Serotonin is a common brain chemical messenger. It is part of the wake promoting system of the brain. Along with other chemical messengers, it helps keeps us awake during the day. It tends to be less active as we fall asleep, when other sleep promoting chemicals take over.

Balance w/melatonin: Melatonin which helps regulate your circadian (light /dark cycle) rhythm works in balance with serotonin. Light shifts the balance to serotonin, -mood and wakefulness darkness shifts it to melatonin-helps you sleep.