Two things which tend to be impacted first are your memory and your IQ, if you are not getting the sleep you need. So your studying will be less effective and it will take longer. I think you might be better off picking things from your textbook and notes to record then listening to the playback while you are doing other things. Get the sleep you need and use time that would otherwise go to waste, like shopping. Or if you can play your recordings back on speakers instead of headphones, while you are driving around.

And I've never managed to get $18/hour - so I think you are doing really well.