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Your internal body clock can determine how deep your gains are.

Are you a morning or evening exerciser? Thanks to research from two new studies, the difference could wipe some time off your personal best if you line it up with the end of your daily circadian rhythm.

A study released in science journal, Cell Metabolism by the Weizmann Institute of Science, Israel on Thursday, and another from the Center of Epigenetics and Metabolism at the University of California, U.S.A have found evidence that no matter how hard you push that morning run, your evening run will run laps around it, and feel less brutal too.

They started by looking at the relationship between time of day and performance in mice, in what was probably the world’s most adorable gyms. And both teams found that the mice ran for longer in the evening part of their mice-y days. No doubt spurred on by a tough day at work chewing through the electrical wires of a house.

The researchers then moved onto humans, whacking them onto exercise bikes at both 8 a.m. and 6 p.m., which is roughly the “active” time of a human’s circadian rhythm. And they found that the 8 a.m. cyclers consumed more oxygen and had higher heart rates – meaning their performance was far less efficient than the 6 p.m. cyclers.

So next time you’re at the gym at 8 a.m., don’t beat yourself up if you’re struggling to lift those weights, just brave the after-work crowd and reap the gains.