We all know we should be working out regularly. But when is the best time to do it? Does it really matter? Well, yes and no. In the long run, staying active is what's most important. After all, less than a quarter of all Americans get the recommended amounts of aerobic activity and strength training, according to the latest Centers for Disease Control figures (PDF). But if you're not a morning person, don't sweat it. Researchers at the University of South Carolina found that we are at our fastest and strongest between 7 pm and 11 pm, thanks to our circadian rhythms. So there's no harm in evening exercise. That being said, there are some serious benefits to getting in your fitness before you start the day. So if you're known to slack off or are worried that your day will run away from you, get your fitness in before breakfast. Plus, there are some quantifiable benefits to exercising in the AM.