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Then go take a nap or something, Went :<
You shouldn't be up if you're tired.
And if you can't nap, then meditate. It restores your energy a lot quicker than taking a nap. I should know. I do it while at church services/mass at my school and it works like a charm. /I also did it while studying for my Forensics test and managed to remember everything I studied despite only looking over it once or twice
Just make sure you're in a tranquil location when meditating, that's all you need.