I'm nocturnal but my beardie is not. I keep the livingroom light on but turn off his lights and he's always consistently out like a light at midnight, regardless of the noise and movement I make at night. Point is, bearded dragons are not nocturnal. They should be sleeping when it's dark for them, or when they perceive it's night time. If he eats at night when he should be sleeping he may feel too stressed during the day so he won't eat until it's night time, or maybe there's something wrong with him, he may be sick..?
I take out any excess food before I turn out the lights because I don't want to encourage mold or rotting. Maybe you can do the same and encourage him/her to start eating when he/she should be?