There's plenty of evidence that athletic performance depends on the time of day (as I discuss in detail in my book!). In general, early evening seems to be the time of maximum performance, which coincides with the point at which your body temperature is highest. But it's still not clear exactly how circadian rhythms affects performance, and which particular aspects are most affected. A new study in the journal Applied Physiology, Nutrition, and Metabolism, from David Hill at the University of North Texas, takes a look a few specific parameters of interest.
The key part of the study involved 20 volunteers doing time-to-exhaustion tests on a stationary bicycle, at a constant power that they were able to maintain for about five minutes. They did it once in the morning (between 6:30 and 9:30 a.m.) and once in the evening (between 5:00 and 8:00 p.m., on a different day). As expected, time to exhaustion was about 20% longer in the evening (329 vs 275 seconds).
So what were the specific aspects of performance that enabled this increase? The most surprising finding is that athletes were actually less efficient in the evening. At a given constant power, they consumed more oxygen (and thus more energy) by about 6%. This negative was outbalanced by a bunch of positive effects in the evening:
- higher VO2max by 4%;
- higher anaerobic capacity (as measured by "maximum accumulated oxygen deficit") by 7%;
- faster VO2 kinetics, which essentially means they were able to ramp up aerobic energy production more quickly, and thus make their anaerobic energy stores last longer;
- higher max heart rate (193 vs 189).
Why does all this happen? Not clear. The paper discusses the fact that circadian rhythms are much more complicated than we used to believe -- there isn't just one clock, there's a "master" clock in the brain (in the superchiasmatic nuclei) that's in constant two-way communication with "slave" oscillators located throughout the body. There's some evidence that muscle contraction may actually be what "sets" some of those slave clocks. As for the role of temperature, higher temperatures speed up the rate of metabolic reactions in the body, though this doesn't explain why the volunteers became less efficient in the evening.
A question the researchers pose for future study: does this mean that athletes should train in the late afternoon or early evening, since they will be able to achieve higher levels of performance? It's worth remembering that research back in the 1980s showed that training at a specific time of day preferentially improves your performance at that specific time of day -- so I'd think the question depends on when your competitions tend to be held. For marathoners, that's a pretty good argument in favor of morning workouts.