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The Potentially Valid Excuse: You Didn't Sleep Well Last Night

You should absolutely skip your workout if: You'd planned to exercise first thing but got a truly terrible night's sleep (we're talking five or six hours of shut-eye, max).

The reason: In a perfect world, you'd get the recommended amounts of sleep and exercise every day, but if you're forced to choose between them, choose sleep. It's essential for a host of health functions (cognitive functioning, appetite regulation, etc.), and your workout may be subpar anyway, if you push it, as better sleep leads to better workouts. Exercising on inadequate sleep could also increase your odds of getting hurt. "If an early-morning client tells me they're really tired, we're not going to do anything that involves complex movement patterns, because I know they're not alert and focused enough to do them correctly or safely," says Jessica Matthews, an ACE senior advisor on health and fitness education and assistant professor of exercise science at Miramar College.

When you can exercise again: As soon as you're back to seven to nine hours of sleep per night.