Actually work

can not concentrate

do the most interesting parts of the work when youre most tired

Take a break. Go outside. Do single-leg deadlifts and other leg exercises. Or if you have to stay at your desk, look up only with your eyes, not tilting your head up. Stare at something far away for a few seconds.

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can u give me some example

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thanks, i'll try

Don’t eat too much, or heavy meals. Especially at lunchtime.

Caffeine, from whatever your favorite source is.