While some people are up at the crack of dawn to lace up their running shoes, others can’t fathom a workout before noon. Finding the perfect time to exercise is as much about personal preference as it is physiology.The effect of training at a specific time of day: a review. Chtourou H, Souissi N. Journal of Strength and Conditioning Research / National Strength & Conditioning Association, 2012, Dec.;26(7):1533-4287. Exercise is supposed to feel good—but if muscles are tight in the morning or working out too late disrupts sleep, it can feel counterproductive.

Afternoon Advantages

Plenty of people tout the benefits of early morning sweat sessions, but if you can’t fit in a workout before noon, don’t sweat it. Research suggests the body could adapt to regular gym dates, so if we hit the weight room every day at 4 p.m., eventually we might perform better at that time than at any other time of day.The effect of training at a specific time of day: a review. Chtourou H, Souissi N. Journal of Strength and Conditioning Research / National Strength & Conditioning Association, 2012, Dec.;26(7):1533-4287. These findings are similar to earlier research, which suggests that sticking to a specific workout time can result in better performance, higher oxygen consumption, and lower perceived exhaustion.Temporal specificity in adaptations to high-intensity exercise training. Hill DW, Leiferman JA, Lynch NA. Medicine and Science in Sports and Exercise, 1998, Jun.;30(3):0195-9131. But scheduling a workout is more complicated than choosing a number on the clock.

Your body’s core temperature is an important factor in determining the quality of exercise. A cold body leaves muscles stiff, inefficient, and susceptible to sprains, whereas higher body temperatures leave muscles more flexible. Body temperature typically increases throughout the day, so muscle strength and endurance may peak in the late afternoon, when body temperature is highest.Different effects of heat exposure upon exercise performance in the morning and afternoon. Racinais S. Scandinavian Journal of Medicine & Science in Sports, 2011, Feb.;20 Suppl 3():1600-0838. The afternoon is also when reaction time is quickest and heart rate and blood pressure are lowest, all of which combine to improve performance and reduce the overall likelihood of injury.

Hormone levels are also important in determining optimal workout time. Testosterone is important for muscle growth and strength, in ladies and gents.Testosterone dose-dependently increases maximal voluntary strength and leg power, but does not affect fatigability or specific tension. Storer TW, Magliano L, Woodhouse L. The Journal of Clinical Endocrinology and Metabolism, 2003, May.;88(4):0021-972X. And the body produces more testosterone during late afternoon resistance training than it does during morning workouts.Interactions of cortisol, testosterone, and resistance training: influence of circadian rhythms. Hayes LD, Bickerstaff GF, Baker JS. Chronobiology International, 2010, Oct.;27(4):1525-6073. Plus, the stress hormone cortisol, which aids in the storage of fat and reduction of muscle tissue, peaks in the morning and decreases throughout the day and during exercise.The cortisol awakening response (CAR): facts and future directions. Fries E, Dettenborn L, Kirschbaum C. International Journal of Psychophysiology: Official Journal of the International Organization of Psychophysiology, 2008, Sep.;72(1):1872-7697. But early birds, take heart: Morning workouts can be successful too.

Morning Perks

It’s sometimes easier to keep a morning workout routine consistent. Afternoon and evening workouts are more likely to conflict with other responsibilities as the day progresses. Plus a full day’s work can take a serious toll on willpower—which can overcome any gym-goer’s best intentions.

Morning workouts might also be a good option for stress-free snoozing. Since exercise increases heart rate and body temperature, working out too late in the evening (generally after 8 p.m.) may disrupt sleep, while one study showed that working out at 7 a.m. (compared to 1 p.m. or 7 p.m.) may help people sleep more soundly at night.Effects of exercise timing on sleep architecture and nocturnal blood pressure in prehypertensives. Fairbrother, K., Cartner, B., Alley, J., et al. Vascular Health and Risk Management, 2014; 10: 691–698.

Finally, one study found that 45 minutes of moderate morning exercise (like walking briskly on the treadmill) helped curb appetite directly after working out.Neural response to pictures of food after exercise in normal-weight and obese women. Hanlon B, Larson MJ, Bailey BW. Medicine and Science in Sports and Exercise, 2013, Feb.;44(10):1530-0315. Research also shows that people can burn up to 20 percent more body fat exercising on an empty stomach—much easier to do first thing in the a.m. than at night.Breakfast and exercise contingently affect postprandial metabolism and energy balance in physically active males. Gonzalez JT, Veasey RC, Rumbold PL. The British Journal of Nutrition, 2013, Jan.;110(4):1475-2662.

The Bottom Line

In the end, it’s most important to find a realistic, consistent workout schedule, no matter what the time. If working out in the morning is best for your schedule, just make sure to warm up muscles that might be cold and tight from sleep. And to keep afternoon workouts consistent, treat them as unbreakable appointments, find a workout buddy, and keep a gym bag in the car or office to minimize excuses.

Originally posted April 2011. Updated July 2015.