Why do you have your lights go ON during the night and OFF during the day?

Wouldnt it be much easier if they were ON during the day so you didnt have to worry about disturbing them? Plus if the OFF period is during the night you wouldnt have to worry about light leaking in.

Im just asking b/c thats how I am planning on doing it if and when I get to 12/12, so is there any reason to do it OFF during the day and ON during the night?