I've found sleep hygiene is a big part of it for me. If I'm on a screen all day and night, right up until bed, I will need 8-9 hours to feel well rested. If I'm screen-free for 2-3 hours before bed, after a night or two I am as rested on 7 hours. And this maintains for as long as I'm good about being screen-free (Kindle included but that's just me perhaps, or psychosomatic maybe). Unfortunately it's not always possible to be away from all screens for that long at the right time, and it seems pretty easy to mess it up for several days with one or two slip ups.