Ideally not. Hair is at its weakest when it is fully wet, and sleeping with just-washed hair can lead to breakage of hair strands, increase in frizz and excess hair fall . Firstly, try never to wash your hair at night. If you don’t have a choice, then dry it or at least wait for it to be only mildly damp before you sleep with it. Also, try and apply a good strengthening leave-in conditioner before bedtime, so your hair is lesser susceptible to breakage.