If your morning workouts feel like wading through wet sand while your late-afternoon sessions feel crisp, you're noticing something the research has documented for decades. For most people, several markers of physical performance — strength, power output, anaerobic capacity, flexibility — tend to be higher in the late afternoon or early evening than first thing in the morning. The leading explanation ties this to the body's daily temperature rhythm.
This article explains why that timing tends to hold, how strongly, and where it stops being a clean rule.
Your body runs on a temperature rhythm
Core body temperature follows a robust circadian rhythm. It reaches its minimum (CBTmin) in the late part of the night, a couple of hours before habitual wake, and its maximum (CBTmax) roughly twelve hours later — for many people in the late afternoon or early evening. This rhythm is one of the most reliable circadian markers we have.
Why would temperature matter for performance? Higher muscle temperature is associated with faster nerve conduction, improved muscle contractile properties, lower joint and muscle stiffness, and more efficient energy metabolism. The late-afternoon window, when body temperature is near its daily high, lines up with when several performance measures tend to be best. The body is, in effect, already partly "warmed up."
What the sports-chronobiology literature shows
Atkinson & Reilly (1996) reviewed circadian variation in sports performance and found that many physical performance variables peak in the early evening, near the body-temperature peak, with the time-of-day swing being meaningful for anaerobic and strength-type measures in particular. Later work, including Facer-Childs & Brandstaetter (2015), reinforced that time-of-day effects on physical performance are real and are shifted by individual chronotype: a person's "peak" is anchored to their own internal clock, not the wall clock, so an early type and a late type don't peak at the same hour.
A few honest caveats keep this from being oversold:
- The effect sizes vary by task. Power and strength measures show the pattern more consistently than, say, fine motor or some endurance measures.
- Training adaptation matters. Performing repeatedly at a given time of day can partly shift your best performance toward that time — so a committed morning trainer may narrow the gap.
- This is about performance output, not a claim about injury, health outcomes, or that any single time is "correct" for everyone.
Movement also nudges the clock — weakly
There's a second, smaller reason timing matters: exercise itself acts as a non-photic zeitgeber — a signal that can shift circadian phase. Youngstedt et al. (2019) mapped a human phase-response curve to exercise: morning exercise tends to advance the clock and late-evening exercise tends to delay it, with effects bounded to roughly an hour per session. This is much weaker than light, which remains the dominant clock-setter, but it's a reason very late, intense sessions may push sleep later for some people.
How Kairo uses this
Kairo estimates where your late-afternoon physical window is likely to fall by anchoring it to your estimated body-temperature peak, which it places relative to your own sleep timing and chronotype rather than a fixed clock hour. It surfaces that as a suggested window for strength or power work — a proposal you confirm, never an automatic change. Crucially, Kairo keeps your physical-readiness estimate separate from your cognitive peak, because they ride different rhythms: your sharpest-thinking window and your strongest-lifting window generally are not the same time of day. As you log workouts, the estimate nudges toward your own pattern. Kairo does not measure your core temperature or promise performance gains; it offers a timing estimate to test.
The takeaway
If you have the flexibility, the late afternoon to early evening is a reasonable default window to try for strength and power work, because it tends to coincide with your body-temperature peak. But "tends to" is doing real work in that sentence: your chronotype shifts the window, training adapts it, and the morning is not "wrong" — it's just often a little harder on the dimensions temperature helps with. Treat the timing as a personal experiment, and if evening sessions are leaving you wired at bedtime, that's the exercise phase-shift talking — pull them earlier.
References
- Atkinson, G., & Reilly, T. (1996). Circadian variation in sports performance. Sports Medicine.
- Facer-Childs, E., & Brandstaetter, R. (2015). The impact of circadian phenotype and time since awakening on diurnal performance in athletes. Current Biology.
- Youngstedt, S. D., Elliott, J. A., & Kripke, D. F. (2019). Human circadian phase-response curves for exercise. The Journal of Physiology.
- Roenneberg, T., et al. (2012). Chronotype and individual differences in circadian timing (MCTQ). Current Biology.
Not medical advice. Kairo estimates circadian timing; it does not diagnose or treat any condition.