The Biophysics of Pre-Sunset Exercise: Why Intense Morning Exercise is Sub-optimal

The study by Souissi et al. found that both continuous and alternated-intensity cycling exercises resulted in better performance metrics, such as time to exhaustion and peak heart rate, when performed in the late afternoon compared to the morning.[1] This suggests that exercise capacity may be enhanced later in the day, potentially due to circadian influences on physiological parameters.

Savikj et al. conducted a randomized crossover trial in individuals with type 2 diabetes and found that afternoon high-intensity interval training (HIIT) was more effective at improving blood glucose levels compared to morning HIIT.[2] Interestingly, morning HIIT was associated with an increase in blood glucose levels, indicating a potential adverse effect when exercise is performed in the morning for this population.

Kang et al. performed a systematic review and meta-analysis, which revealed that endurance performance, as measured by time-to-exhaustion, was higher in the afternoon than in the morning.[3] This finding aligns with the notion that circadian rhythms may favor physical performance later in the day.

Galan-Lopez and Casuso also conducted a systematic review and meta-analysis, which indicated that afternoon exercise was more effective at reducing circulating triglycerides and potentially fasting blood glucose levels compared to morning exercise.[4] This suggests that metabolic adaptations to exercise may be more favorable in the afternoon.

Mirizio et al. noted that short-duration maximal exercise performance peaks in the late afternoon, which may be influenced by factors such as body temperature and hormone levels that follow a circadian pattern.[5]

Finally, Youngstedt et al. explored the circadian phase-response curves for exercise and found significant phase advances with afternoon exercise, which could have implications for optimizing exercise timing to align with circadian rhythms.[6]

Overall, the evidence suggests that from a circadian perspective, heavy exercise in the afternoon may be more beneficial than in the morning, as it aligns better with physiological and metabolic rhythms that enhance performance and metabolic outcomes.

References

1. Souissi A, Yousfi N, Souissi N, Haddad M, Driss T. PloS One. 2020;15(12):e0244191. doi:10.1371/journal.pone.0244191. The Effect of Diurnal Variation on the Performance of Exhaustive Continuous and Alternated-Intensity Cycling Exercises.

2. Savikj M, Gabriel BM, Alm PS, et al. Diabetologia. 2019;62(2):233-237. doi:10.1007/s00125-018-4767-z. Afternoon Exercise Is More Efficacious Than Morning Exercise at Improving Blood Glucose Levels in Individuals With Type 2 Diabetes: A Randomised Crossover Trial.

3. Kang J, Ratamess NA, Faigenbaum AD, et al. Journal of Strength and Conditioning Research. 2023;37(10):2080-2090. doi:10.1519/JSC.0000000000004497. Time-of-Day Effects of Exercise on Cardiorespiratory Responses and Endurance Performance-a Systematic Review and Meta-Analysis.

4. Galan-Lopez P, Casuso RA. Sports Medicine (Auckland, N.Z.). 2023;53(10):1951-1961. doi:10.1007/s40279-023-01879-0. Metabolic Adaptations to Morning Versus Afternoon Training: A Systematic Review and Meta-Analysis.

5. Mirizio GG, Nunes RSM, Vargas DA, Foster C, Vieira E. Scientific Reports. 2020;10(1):9485. doi:10.1038/s41598-020-66342-w. Time-of-Day Effects on Short-Duration Maximal Exercise Performance.

6. Youngstedt SD, Elliott JA, Kripke DF. The Journal of Physiology. 2019;597(8):2253-2268. doi:10.1113/JP276943. Human Circadian Phase-Response Curves for Exercise.