you should take a at list a 7-8 hours sleep everyday,this rest need your body,otherwise you feel uncomfortable,you take a proper sleep then you feel good and fresh,and enjoy your work.and totally feel relax,it help you do it.
i usually get 6-7 hours but recently its been a little less cause I stay up late to watch dramas and stuff....its summer, thats what u do! during school i'd get more though.