The best time to study is before 12 am and from 4 am to 7 am. But whenever you study you must study with full concentration, devotion, and an inner urge to study. Try to block all the things or keep off which may cause distraction.
You can read anything at any time as long as you are in a well-lit room, so that you don't end up with vision problems. When studying, I always listen to music, 'cause it helps me keep motivated enough and it makes things much easier for me to memorize.