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To relax so as to recover, recuperate, regain one's energy. Be sure to rest up tonight—tomorrow's game is going to take everything we've got! We pushed ourselves to the limit on that last project, so the whole team needs a bit of time to rest up.
rest up (from something)
to recover or recuperate from something tiring. I need about a week to rest up from my long travels. I'll need a few days to rest up.
To relax and replenish energy in preparation for later strenuous activity: The baseball players rested up for the big game.