The Best Workouts to Do When You're Too Tired to Work Out

Why you're tired: You've been sitting all day.

What to try: A short high-intensity interval workout (HIIT —for example, all-out cycling bouts for 30 seconds, followed by 4 minutes of rest, repeated 8 times).

Why it could help: Plenty of research has proved that even a single bout of moderate-intensity exercise can boost energy more than not exercising. In one study, volunteers reported feeling more jazzed after their workout than after eating a candy bar. Here's what scientists think is going on: Exercise alters the levels of one or more energy-boosting neurotransmitters (like dopamine or serotonin) in the brain, explains Patrick O'Connor, PhD, a professor in kinesiology at the University of Georgia who has headed some key studies on exercise and fatigue. He adds that these types of short HIIT routines can increase the feeling of vitality without tiring us out the way longer workouts might.

Why you're tired: You've been sitting all day, every day for months.

What to try: Invite a friend to join you for a brisk walk that lasts at least 30 minutes.

Why it could help: No surprise —people who work out regularly report feeling more energetic than those who don't, research shows. We don't really know the specifics of why, O'Connor says, but it could be a combination of the hits of energy they get from each bike ride, stair climb or hustle across town, as well as the deeper, more restful sleep they earn as a result of having used their muscles. Walking is often recommended as the gateway drug of exercise because it has the lowest risk of injury and sudden cardiac arrest —yet it’s still considered moderate-intensity if you stride with purpose. What's more, sports psychologists have found that one of the most common reasons people (especially women) quit working out is that they felt like they weren't doing the exercises well enough to make a difference. But walking is a beautifully egalitarian exercise that has the power to make anyone feel accomplished.

Why you're tired: You've been working out too much.

What to try: Do something easy that gets you going, but not so much that it makes you sweat or pant: Play Frisbee with your kids or dog, take an easy yoga class or ride your bike to the movies.

Why it could help: Overtraining is caused by a lack of recovery time, and it can put you at risk of injury. Contrary to what most of us expect, though, experts say that the best kinds of "rest days" involve light, non-challenging activity that enhances circulation. By getting the blood flowing in this way, you'll heal better, get more nutrients to your body and allow your muscles to regenerate faster than if you had spent the time "resting" in front of the TV.

Why you're tired: You didn't get to bed until 3 a.m.

What to try: Skip your workout. Go to sleep.

Why it could help: If you didn't get enough rest last night, a 20-minute afternoon nap can help you get through the day. Getting a full 7 to 8 hours tonight will help you feel better tomorrow—strong enough to rock any workout you choose.