There are 2 pathways in your brain — a wake pathway and a sleep pathway. Each pathway has neurotransmitters that send signals to determine whether you are awake or asleep.
Under regular conditions, sleep occurs when the wake neurotransmitters turn down and sleep neurotransmitters ramp up and take over, allowing you to fall asleep and stay asleep.
There are many neurotransmitters in the wake and sleep pathways that work together to determine whether you are awake or asleep. Orexin is one of the wake neurotransmitters.