Whether it’s hitting up SoulCycle or taking a run in the park, exercise is key to stress management, good sleep, happiness, productivity — and above all, health. But when should you push yourself the hardest, and when should you take a break? Which excuses to skip a workout are valid, and which aren’t?

It’s important to distinguish between “overreaching” and “overtraining,” Jimmy Bagley, Ph.D., assistant professor of kinesiology and director of the muscle physiology lab at San Francisco State University tells Thrive. Overreaching is what you want to be doing, Bagley notes — you work a little harder each day, week, or month, slowly maximizing your goals. As a result, you cause “overload” in your body (in a good way), which leads to adaptation and, ultimately, improvement. Conversely, overtraining is when you don’t give your body enough time to recover between workouts.

We’ve rounded up six of the most common excuses for skipping physical activity (think: because you had a stressful day at the office), along with tips on whether you should work out or chill out for the sake of your well-being:

The excuse: “I’m stressed.”

The verdict: Work out anyway.

Exercise helps relieve your stress. Rather than being an excuse for why you don’t work out, stress should be a reason for why you do hit the gym.

“Exercise is physiological: Your blood flow to the brain increases, your breathing rate increases, and your heart rate increases. All of this is great for how you will feel a couple hours after completing an activity,” Bagley tells Thrive. “When you exercise, your body releases endorphins, and those go to your brain. They stimulate the sensors in your brain that are related to rewards.” 

This means that even if you enter the gym stressing over a big work project, your brain chemistry will help you feel happier by the end of your sweat session — and it will continue for hours afterward. So yes, you should go to that spin class — especially if you’re feeling overloaded. Your brain will thank you.

The excuse: “I don’t feel like it.”

The verdict: Lighten up.

If you aren’t in the mood for an intense HIIT class, that’s okay. But it’s worth choosing an easier, more manageable alternative.

“If you’re just not feeling it — say, if you have kids at home and they were up all night — you don’t have to necessarily stick to your plan for that day. You can do something else lighter,” Bagley says.  Examples of lighter exercise that you can do instead include walking slowly (less than 2 m.p.h.), bicycling, and light yoga (like yin, not vinyasa power flow).

The key is to not view each workout as independent, Bagley explains. Look at your year-long plan and say, “I’ve got 100 training sessions this year. Is it okay for me to take this day, and not make it a super challenging workout?” In the grand scheme of things, one day won’t change things. Just keep the bigger picture in mind.

The excuse: “I’m sore.”

The verdict: It depends.

How long after your last workout does your soreness continue? Your answer to that question will determine whether to go hard or take it slow. “The day of your exercise, if you’re a little sore, that’s fine, but if you’re still sore two to four days after you’ve exercised, you’ve got delayed onset muscle soreness (DOMS),” Bagley tells Thrive.

If you’re experiencing manageable next-day soreness, go for it, and don’t let that alter your planned exercise. But if you have DOMS, Bagley says that working very hard is actually detrimental: Your muscles are inflamed and trying to repair themselves, so training intensely on top of that would damage the muscles and delay recovery. Instead, turn down the dial and rest. Pushing through would be overtraining, so don’t go all out.

The excuse: “I’m tired.”

The verdict: Push through, but listen to your body.

Mild fatigue isn’t a reason to ditch the running shoes, Bagley says. This is a case when you should push through. Exercise can actually help increase your energy, so it’s worth going through with it. That said, you want to be careful not to do too much if you’re truly very tired, and should pay attention to how you feel throughout your workout. “Listen to what your body is telling you,” he suggests.

The excuse: “I’m really tired.”

The verdict: Skip it.

Fatigue is one thing, but full-blown sleep deprivation should be an exercise red flag for you. Bagley takes this distinction seriously. “Sleep deprivation has a lot of physical and mental effects. Physiologically and psychologically, exercising is probably going to be detrimental to your health that day,” he tells Thrive. If you got very little sleep last night, know that taking the time to rest is more beneficial than your workout.

The excuse: “I don’t have enough time.”

The verdict: Get creative.

You definitely don’t need a two hour session at the gym to see the mental and physical benefits of exercise. Bagley cited this excuse as one of the most common among his clients during his time as a personal trainer.

According to The Department of Health and Human Services, 150 minutes of exercise a week is optimal, which is equivalent to 30 minutes, five days a week. “Any amount of moderate to vigorous activity can add up to 30 minutes. It doesn’t need to be a structured program,” Bagley tells Thrive. “You can spread it out.” You can try three 10-minute walks a day, or two 15-minute quick in-home workouts, one in the morning, and one in the evening.

Follow us here and subscribe here for all the latest news on how you can keep Thriving.

Stay up to date or catch-up on all our podcasts with Arianna Huffington here.