Sleepy? A lack of shuteye reduces the amount of growth hormones needed to boost your immune system and build muscle. That can be counterproductive. If you get to bed late, it’s better to postpone your morning workout and sleep in.

Tired? Fatigue can be a sign you should take a day off. “But if you are tired from a long day and had adequate sleep, sometimes a workout can actually give you energy and make you feel better,” explains Heather Gillespie, MD, MPH, of UCLA’s sports medicine division.

Injured? Different injuries have different requirements, so do some research first. The biggest mistake Gillespie says she sees is that people with injuries “return too soon to the activity that caused their injury, or they begin exercising too soon [after an injury].”

Sick? If you don’t have a fever with headache, runny nose, nasal congestion, sneezing and sore throat, then exercise in moderation. But if fever is accompanied by aches, chest congestion, stomach cramps or vomiting, wait 48 hours after symptoms disappear before exercising.