Many sleep scientists say that the key to healthy sleep is a darkened room. As in, all lights off, all light sources covered (including phones). 

All of this allows the brain to perceive (as our ancestors did) darkness as a cue to fall asleep. 

And this is the biggest benefit: it is so damn soothing. It feels a bit like “darkness” therapy as you lie there, eyes open the last minute before you conk out, staring into the exact same darkness that exists behind your eyes, and will eventually lull you to sleep.