The best time of day to exercise depends on your goals

Much like the timing of when you eat, the time of day that you exercise may be an important factor in getting the results you want, according to two recent studies both published in Cell Metabolism. To lose weight, for example, exercising in the morning may be better. Conversely, to achieve optimal physical performance, evening exercise may be more beneficial.

In the first study, researchers from the University of California, Irvine (UCI), Irvine, CA, concluded that morning exercise may confer better results than nighttime exercise for getting rid of extra pounds.

"Exercise stimulates metabolism, leading to the improvement of metabolic health. While the metabolic benefits from exercise have been extensively uncovered, the question of when it is appropriate to exercise has remained virtually unexplored," said senior author Paolo Sassone-Corsi, PhD, Donald Bren Professor, Department of Biological Chemistry, and director, Center for Epigenetics and Metabolism, UCI School of Medicine. "Using mice, we compared the impact of exercise on the skeletal muscle metabolism at different times of day. We discovered that exercising at the correct time of day—around mid-morning—results in more oxygen in the cells and a more rejuvenating effect on the body."

Dr. Sassone-Corsi and colleagues used high-throughput transcription and metabolomics to pinpoint metabolic changes in mice during exercise, comparing results from evening exercise sessions with morning ones. They found that exercising in the morning used more carbohydrates and ketone bodies, and resulted in increased fat and amino acid breakdown.

Thus, exercising in the morning will result in losing more weight compared with exercising later in the day because you are burning more carbs and breaking down more fat.

Dr. Sassone-Corsi concluded: "Our results clearly indicate that time-of-day is a critical factor to amplify the beneficial impact of exercise on both metabolic pathways within skeletal muscle and systemic energy homeostasis."

Results from a second study, also published in Cell Metabolism, complemented findings from Dr. Sassone-Corsi et al. In this study, researchers concluded that exercising in the evening may be more effective in achieving optimal physical performance.

Senior author, Gad Asher, MD, PhD, professor, Department of Biomolecular Sciences, Weizmann Institute of Science, Rehovot, Israel, and his colleagues found that overall exercise performance of mice was significantly better during the “mouse evening,” toward the end of their active time, compared with “mouse morning.” In fact, it was about 50% better on average during their evening, and even more in some testing.

Like Dr. Sassone-Corsi and his colleagues, Dr. Asher et al used high-throughput transcriptomics and metabolomic studies, but on muscle tissue. During evening exercise, the mice had higher levels of 5-aminoimidazole-4-carboxamide ribonucleotide (ZMP), a metabolite known to activate metabolic pathways related to glycolysis and fatty acid oxidation via the activation of AMP-activated protein kinase (AMPK)—an enzyme present in all mammalian cells. Exercise activates AMPK in skeletal muscle, stimulating energy-generating processes (including glucose uptake and fatty acid oxidation), and decreasing processes that consume energy (including protein and lipid synthesis).  

Due to higher ZMP levels, the mice had an increased exercise capacity in the evening. In further studies that included 12 human volunteers, Dr. Asher and colleagues found the same effects. Human study participants all exhibited lower oxygen consumption during evening exercise compared with morning exercise, which resulted in better exercise efficiency.

By consuming less oxygen, the body will have more oxygen available to create energy and require less effort during training. This translates into better exercise efficiency, as well as increased exercise capacity, giving you the ability to go longer in your workout or run in the evening compared with in the morning.

These results differ from, but complement, the conclusion of Dr. Sassone-Corsi et al: that exercise was most impactful on the metabolism in late morning compared with evenings, in human subjects.

Both researchers, however, agree on the importance of circadian rhythm.

"It's quite well known that almost every aspect of our physiology and metabolism is dictated by the circadian clock. This is true not only in humans but in every organism that is sensitive to light," concluded Dr. Asher.

Dr. Sassone-Corsi agreed: "Circadian rhythms dominate everything we do. Previous studies from our lab have suggested that at least 50% of our metabolism is circadian, and 50% of the metabolites in our body oscillate based on the circadian cycle. It makes sense that exercise would be one of the things that's impacted."

To put these results into the context of your day and your goals: If you want to exercise to lose weight and be rejuvenated, late morning would be the best time for your workout. Conversely, to achieve optimal performance, say increase your 5.5-mile run to 6 miles, or up your benching from 175 to 180 lb, do it in the evening. Your body may thank you—and surprise you.

The study conducted by Dr. Sassone-Corsi et al was supported by the Novo Nordisk Foundation, the Swedish Diabetes Foundation, Swedish Research Council, the National Institutes of Health, INSERM, DAPRA, the National Institutes of Health, and the Della Martin Foundation. The study conducted by Dr. Asher and fellow researchers was supported by the European Research Council, the Fonds de Dotation AGIR pour Les Maladies Chroniques, and by the French National Research Agency.