Fish will sleep when the lights are low or completely off. When the light is on that is when they are the mst active. That being said sometimes when the lights are still on my fish are ready to go to sleep and go to thier respective places in the tank where they always sleep. Some of it is just up to the fish and when they feel like sleeping.
I got some marble hatchetfish yesterday and apparently at the fish store bedtime is about 8pm. So at 8pm all six of them went to the top right hand corner and waited. Unfortunately for them, bedtime is not until 10pm at my house!