Some scientists agree that late afternoon is the optimal time to exercise because of your body's circadian rhythms. Circadian rhythms control your body temperature, which is crucial to an effective workout. Circadian rhythms dictate that your body temperature is highest in the late afternoon, between 2 and 4 pm. Also, you're more alert in the afternoon and your muscle strength is at its highest. These translate into improved performance and a decreased risk of injury.
Others in the health field say exercising in the morning is best because you're revving your body's metabolism early and will feel more energetic throughout the day. Some studies do show that your body burns more fat when you exercise after fasting, which is generally only possible early in the morning. However, exercising on an empty stomach in the morning may be counter-productive because you cannot rely on carbohydrates for energy to fuel your workouts. This could translate into becoming fatigued sooner, failing to exercise as vigorously or for as long, and burning fewer calories overall when compared to exercising later in the day after you've eaten and your body is properly fueled. Plus, the slightly higher amount of fatty acid oxidation is so insignificant that it does not translate into a large number of extra calories burned (especially if you have to cut your exercise session short because you're too tired to finish).
Remember to take the results of periodic studies with a grain of salt. First of all, it's important to look at how the study was done. Take note of the number of participants involved and the methods used in the study. If the researchers studied only a small number of people, those results are less credible than a study involving hundreds or thousands of subjects. Additionally, the results of a study have to be able to be reproduced in order for them to have any credence. When looking at research, remember that it often takes years, even decades, of repeated research studies to establish concrete recommendations based on the results.
The bottom line: Exercise at whatever time of day you're most likely to do so consistently. If you are more likely to stick to an exercise regimen by making exercise a priority at the beginning of your day (before distractions and fatigue from the day's events prevent you from exercising at all), then commit to breaking a sweat in the morning. If evening works best with your schedule, stick to evening exercise. It's not important when you exercise--just make sure you are exercising regularly.
Kari Hartel, RD, LD is a Registered Dietitian and freelance writer based out of St. Louis, MO. Kari is passionate about nutrition education and the prevention of chronic disease through a healthy diet and active lifestyle. Kari holds a Bachelor of Science in Dietetics from Southeast Missouri State University and is committed to helping people lead healthy lives. She completed a yearlong dietetic internship at OSF St. Francis Medical Center in Peoria, IL, where she worked with a multitude of clients and patients with complicated diagnoses. She planned, marketed, and implemented nutrition education programs and cooking demonstrations for the general public as well as for special populations, including patients with cancer, heart disease, diabetes, Alzheimer's disease, obesity, and school-aged children. Contact Kari at KariHartelRD@gmail.com.