Editor's Note: (The following is adapted from "Fitter Faster: The Smart Way to Get in Shape in Just Minutes a Day" by Robert J. Davis with Brad Kolowich Jr. )
(CNN) Some fitness gurus recommend working out first thing in the morning because that's when you're least likely to have scheduling conflicts and therefore more likely to exercise regularly. Plus, early exercisers often say that a morning routine leaves them feeling more energized and productive during the day.
But if you are not a morning person and shudder at the thought of getting out of bed for a 6 a.m. workout, I have good news: We perform best at exercise (especially high-intensity exercise) later in the day.
Research shows that strength and flexibility are greatest in the late afternoon and that perceived exertion -- meaning how hard you feel that your body is working -- is lowest. Scientists attribute these effects to our circadian rhythm, the body's 24-hour clock, which causes body temperature to rise slightly throughout the day and peak in late afternoon.
Of course, none of this means that you're doomed to a subpar workout if you exercise in the morning. By doing so consistently, you can eliminate the morning performance gap, according to research, which shows that athletes who train in the morning improve their performance to levels seen in the afternoon. That's worth keeping in mind if you're planning to run, say, a 5K with a 7 a.m. start time. Your performance will be best if you train at that hour.
Some people do aerobic exercise first thing, before they've eaten, because they think it will help them burn more fat. Indeed, there's some evidence that this practice, sometimes called "fasted cardio," may boost fat burning, but only fleetingly. Over the course of days or weeks (which is what counts), research shows that it doesn't seem to offer any advantages.
For example, in a four-week trial that randomly assigned young women to either fast or drink a 250-calorie shake before their aerobic workouts (while otherwise eating a low-calorie diet), both groups lost the same amount of fat and weight. Similarly, a study involving overweight women who did high-intensity interval workouts for six weeks after either fasting or eating found no differences in fat loss.
Because exercise revs up your body, conventional wisdom has it that working out in the evening interferes with sleep. But overall, research has failed to support this assertion. For example, a small study of young adults found that doing vigorous aerobic exercise two hours before bedtime did not impair their ability to fall asleep or sleep soundly. Likewise, a study involving older people showed that low-impact aerobic workouts done between 7 and 8:30 p.m. were just as effective as morning workouts at improving their self-reported sleep quality.
Of course, everyone is different, so it's possible that nighttime exercise may make it harder for you to sleep. But the only way to know for sure is to try.
All in all, the best time to work out is whenever you can. If you exercise at different times of the day, be sure to note the hour as you're tracking your progress. That way, you'll know when your body clock may be to blame for a less-than-optimal workout.