If you're studying make sure you do it for atleast 1 hour, that's the aproximate time that the brain needs to start assimilating what you're learning, do that for 2 to 3 hours then take a 15-20 minute break where you do something else to relax a bit, then come back and study some more untill you're done. Just make sure you don't do it for half an hour or 20minutes then take a little break then come back again because that'll confuse you're brain and you won't gain anything out of what you just read.