Another important factor that seems to be completely up in the air is the best time of day to work out . "What I recommend is really just whenever you can fit it in; I don't think there's a clear-cut optimal time of day," Dr. Kline says. There used to be a common belief that you shouldn't work out before bed, because it will disturb your sleep, but in the past couple years that recommendation has changed, he says. "Some people are much more reactive to late-night exercise, in the sense that it activates them and they're aroused," he says. "But I think for most people, late-night exercise shouldn't be a concern."