Mollies acting weird!

So, my tropical tank is all set up for about 6-8 weeks now. I have 4 mollies, 2 silver, an orange and a black. The mollies have been acting very weird lately. During the day the mollies are swimming fine around the tank but in the evening they kinda (die?) they just lie at the end in the gravel not moving. Not all of the mollies do this but last night the orange one did it, this morning the black molly did it and the orange has did it again this evening. As soon as I add food to the tank the molly that I thought was dead at the bottom of the gravel start moving again and came up to the surface for food, he then starts swimming around the tank with the other fish normally. Is this normal? whats wrong with them?
:-/


Thanks

 

normal behavior guppies will do the same thing