When is the best time to exercise? What circadian science actually says

For years, the debate has raged between the "up at 5am, workout before coffee" camp and the "late afternoon is when the body performs best" camp. In October 2025, the American Heart Association published a scientific statement that clarified more than most headlines have: timing does matter, but not in the way most social media suggests.

The AHA statement, drawing on decades of chronobiology, concluded that endurance, muscle strength and physical performance genuinely peak in the late afternoon and early evening, when core body temperature, neuromuscular efficiency and muscle function naturally reach their highest levels. However, the same statement was careful to note that observational studies examining exercise timing and cardio-metabolic outcomes remain conflicting (Knutson et al., 2025). A 2024 meta-analysis in Chronobiology International, pooling five randomised trials, found no significant difference in 24-hour continuous glucose readings between morning and afternoon or evening exercise (Dighriri et al., 2024). Translation: for day-to-day blood glucose control, when you exercise probably matters far less than whether you exercise.

Where timing does clearly matter is your body clock. Exercise is now recognised as a secondary zeitgeber, or time cue, helping reinforce the body's internal clock alongside morning light exposure and regular meal timing. A 2020 study in JCI Insight worked with 52 sedentary adults and found that morning exercise advanced the internal circadian rhythm, which may help late chronotypes, often called "night owls", who want to shift towards an earlier schedule. Evening exercise delayed circadian timing further, something that may suit people who naturally prefer later schedules but could make it harder for early chronotypes, or "morning larks", to fall asleep at their usual time (Thomas et al., 2020).

Why this is genuinely liberating

You do not need to force yourself into an unnatural time slot. The best exercise is the one you will actually do, on the days you can do it, at an intensity you can recover from. Once that is in place, small circadian-informed choices can layer on genuine additional benefit.

Practical takeaway: a quick chronotype-based guide

If you naturally wake early and feel sharpest before 10am, you are likely an early chronotype. Morning or early afternoon workouts fit your rhythm best.

If you feel sluggish for the first hour or two and hit your stride in the afternoon or evening, you are likely a late chronotype. Late morning to early evening exercise will often feel more natural, making it easier to stay consistent. If your goal is to shift your body clock earlier, earlier morning exercise combined with bright morning light has the strongest evidence.

If you sit somewhere in between, aim for mid-morning to late afternoon and pay attention to how you feel.

Three universal principles hold, whatever your chronotype:

• Avoid vigorous exercise within one to two hours of bedtime if you notice it delays your sleep. Gentle or moderate movement is unlikely to have the same effect.

• Get outside within an hour of waking. Morning light is the strongest signal for setting your circadian rhythm and can also support better sleep and daytime alertness.

• Consistency beats perfection. Three or four moderately timed sessions a week outperform one occasional "optimally timed" workout.

Best practice

Pair your workout with a fixed daily anchor, such as after school drop-off, before your first afternoon meeting or straight after work. Habits are far more likely to stick when they are linked to reliable time markers rather than good intentions.

References:

Knutson, K. L., Dixon, D. D., Grandner, M. A., Jackson, C. L., Kline, C. E., Maher, L., Makarem, N., Martino, T. A., St-Onge, M.-P., & Johnson, D. A. (2025). Role of circadian health in cardiometabolic health and disease risk: A scientific statement from the American Heart Association. Circulation, 152(21), e408-e419. 

Thomas, J. M., Kern, P. A., Bush, H. M., McQuerry, K. J., Black, W. S., Clasey, J. L., & Pendergast, J. S. (2020). Circadian rhythm phase shifts caused by timed exercise vary with chronotype. JCI Insight, 5(3), e134270. 

Dighriri, A., Timraz, M., Rosaini, N. D., Alkhayl, F. F. A., Boyle, J. G., Logan, G., & Gray, S. R. (2024). The impact of the time of day on metabolic responses to exercise in adults: A systematic and meta-analysis review. Chronobiology International, 41(11), 1377-1388.