"It's quite well known that almost every aspect of our physiology and metabolism is dictated by the circadian clock," said Gad Asher of the Department of Biomolecular Sciences at the Weizmann Institute of Science. And according to Dr. Steven Feinsilver, director of sleep medicine at Lenox Hill Hospital in New York, “Growth hormone is produced in the middle of the night while cortisol is produced in the early morning.”

That said, it makes sense that certain times of day might work better than others for meeting specific exercise goals. A morning workout boosts the metabolism (sugar and fat) and may aid weight control, while exercising at night uses less energy and oxygen so you can do more for muscle gain and general aerobic health.

But the best workout is the one you do!

AM WORKOUTS

Working out in the morning is the best way to burn stored fat, making it ideal for weight loss. That’s largely because the body’s hormonal composition in the morning is set up to support that goal, says Anthony Hackney, a professor in the department of exercise and sport science at the University of North Carolina Chapel Hill.

”People naturally have elevated levels of cortisol and growth hormone in the morning—both of which are involved in metabolism—so you’ll “draw more of your energy from your fat reserves,” Hackney says. That can potentially help with weight loss.

In addition, an AM workout can help promote activity throughout the day; increase your productivity by improving focus and concentration; assist in blood sugar regulation by increasing the ability of your muscles to metabolize sugar and fat more significantly than evening workouts; and, according to the Journal of Physiology , working out in the morning can improve your quality of sleep.

But if you’re really not a morning person, don’t force it. According to Hackney, “You may be exercising, but it may be at such a low intensity level that you’re really not expending a lot of energy.”

PM WORKOUTS

The body's ability to perform is at its peak is in the afternoon. From 2 pm to 6 pm, your body temperature is at its highest. As your body temperature increases, it optimizes your overall physical performance to include muscle function and strength, flexibility, power output, and endurance. This means that your body is most prepared to exercise at this time, making it an effective time of the day to workout, so consider this permission to hit the snooze button in the morning!

Plus, people who exercise in the evening take up to 20% longer to reach the point of exhaustion, which is more good news for night owls! In addition, working out in the evenings can increase your physical capacity, aerobic capacity, and strength output by between 8% - 30%.

Researchers are finding more details on the connection between exercise efficiency and time of day. There isn’t enough data to recommend exercising at one time or another, so when it comes to the best time to workout the most important factor is considering your own habits.

Because at the end of the day the most effective exercise is the one you do! Doing exercise beats no exercise at all— but only if you opt to keep doing it.

Are you an AM or a PM? Let us know in the comments below.