I find that at night, they like to sleep on something that is near the surface.
In my tanks that do not have live plants, I use Tetra water wonder plastic plants. They will float at the surface if you remove the plastic thing attached to them that is supposed to go into the gravel. At night I will find the bettas resting in the plant. Plus it gives them some cover at the surface and that will make them feel more secure as well.
Or you can use real floating plants depending on how much light the tank gets. I think hornwort and anacharis are both low light floating plants.