Okay, this is going to sound very silly, and it will be if anyone sees you: stand up, stretch upward, breathe in deeply, then, as you breathe slowly out, say 'mmmmooooosssssaaaaahhhhh' and come back to resting position. This will give you the short brain-break you need. Then go get some sleep!
Let me guess. You're in the AP Physics boat like I am (or another of the APs)
Ask what ? Little late !