Time-of-day performance peak

Why are so many records set in the early evening rather than in the morning?

An athlete on the morning of a race is rested, unfatigued, fed and fresh. By six in the evening they have been awake for twelve hours, walked around, eaten twice and burned through a day. On any straightforward accounting the morning should be the better moment — and yet sprint, jump and strength bests cluster in the late afternoon and early evening.

So either "rested" is not the variable that matters most, or something else is moving underneath it. What could be varying across the day that a night's sleep does not simply restore?

Start with the most mechanical thing that changes on a daily cycle. Core body temperature is not constant: it runs on an internal rhythm of roughly a day, trough in the small hours, peak in the late afternoon or early evening, with a swing of around half to one degree. That is not a response to activity — it persists in people kept awake in constant conditions, which is how we know it is a clock rather than a consequence.

Now ask why half a degree should matter to a sprinter. Consider what warmer tissue does. Enzyme reaction rates rise with temperature. Nerve conduction is faster. Muscle is less viscous, so less of each contraction is spent overcoming internal resistance, and the rate at which force can be developed goes up. None of these is a dramatic effect on its own; together, in the same direction, they shift maximal power by a few percent. A few percent is nothing to a jogger and is the whole margin in a hundred metres.

Ask next which events should be most affected. If temperature and neuromuscular readiness are doing the work, the sensitivity should be greatest in short, maximal, technically demanding efforts, and smaller in long endurance events where pacing and fuel dominate. That is broadly the pattern reported — which is a good sign, because a proposed mechanism that predicts where an effect should be strongest is worth more than one that merely fits the average.

Then the morning side of the argument. Waking is not a return to baseline; it is the bottom of the cycle plus a stiffness of its own. Joints and connective tissue are less compliant after a night's immobility, and the body is climbing out of a temperature trough. That is why a morning warm-up has to be longer to reach the same state, and why a very thorough one narrows the gap without closing it.

Now the part that intellectual honesty demands we raise before the story feels finished. We began from an observation about records, not about a controlled experiment. When are finals held? In the evening, because that is when spectators and broadcasters are available. If almost every opportunity to set a record is scheduled at seven in the evening, records will occur at seven in the evening whether or not the athlete is any better then. The observation and the mechanism are entangled, and the observation alone cannot establish the mechanism.

So how would you separate them? Bring the same athletes into a laboratory and test them repeatedly at different hours, with warm-up and nutrition matched. That work has been done, and the diurnal variation in maximal strength and power survives it — smaller than the raw record distribution suggests, but real. Both explanations are true at once, and the scheduling one is probably the larger contributor to the pattern you notice in the record books.

One more complication, which is where the field is currently moving. The relevant clock may not be the one on the wall. Evidence increasingly suggests the peak sits a fairly fixed number of hours after an individual's habitual waking time, so a late chronotype's best hour is genuinely later than an early riser's — and athletes who habitually train in the morning shift some of their advantage toward it. I would treat the precise size and stability of that shift as unsettled rather than established.

The misconception to correct is that this is about accumulated tiredness — that mornings are worse because you are still waking up and evenings are better because you have "warmed up" over the course of the day. Activity does contribute, but the rhythm is not made of it. Keep someone in constant light with no activity and no sleep, and the temperature cycle continues on schedule. The daytime rise is driven, not merely caused by moving about.

A second refinement: the effect is modest and it is not universal across sports. For a marathon, fuel status, heat load, and the weather at the start line matter far more than a fraction of a degree of core temperature. The evening advantage is largest exactly where the margins are smallest, which is also where it is most visible — and that visibility makes it easy to overrate.

• How quickly does the peak shift after travelling across time zones, and what does that cost?
• Does habitually training in the morning move the peak, or only reduce the morning penalty?
• Why are endurance events so much less sensitive to the time of day than sprints?

Every term the collection defines is gathered in the glossary.