I'm pretty sure this is the opposite of what he should be doing :/
I had a great betta who recently passed -And always slept at night. He was also very feisty and always flaring at my fingers, which I loved - the new guy is very passive and only comes out of his cave in the evening. ..Does this happen sometimes or could he be sick? Maybe the lights in the petstore got him off schedule?
I think they're all different. *most* of the ones i've had sleep at night, but the one I have right now sleeps like a guinea pig, you never actually catch him sleeping no matter what time you look or flip on the light.