I don’t see the harm in it if you are not using the too much and you are treating your hair with the correct methods in between. I usually spray a little on my hair before bed when I’ve just done my hair at night. I personally like the slept-in look and it helps to tame the odd bits. It’s not the same as when you sleep with make-up on though because where your skin still breathes, your hair has no breathing ability once it leaves your scalp, so don’t worry too much. Just make sure you take good care of your hair in between.