I would set a timer to either remind myself to take a break or wake me up.

When the timer goes off get up and walk around then settle back down to study and set the timer again. Maybe set it for every 30 minutes or an hour to give yourself a break. Walking around should help you wake up and stretch. I hope this helps. It's the only thing I could come up with that might work on me.
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Bipolar I, Depression, GAD Meds: Zoloft, Zyprexa, Ritalin
"Each morning we are born again. What we do today is what matters most." -Buddha