A study from the Universit de Toulon-Var in France shows that it takes longer to recover from hard exercise in the evening than in the morning (International Journal of Sports Medicine, Volume 27, 2006). Cyclists performed ten six-second bouts of all out effort, with 30 second rest periods while the researchers measured peak power output, total mechanical work, peak pedaling rate, and peak efficient torque. The same group of cyclists performed these workouts in the morning on one day, and in the evening on another day. They found that the short-term recovery patterns were slower in the evening than in the morning..

While the researchers offered no explanation, decreased muscle performance late in the day may have a lot to do with brain function. Each muscle is made up of millions of individual muscle fibers. Each muscle fiber is instructed to contract by a single nerve fiber that receives messages from the brain. Your brain is far more alert after sleeping and napping than after being active for many hours. For example, students score higher in exams taken shortly after waking up than later in the day, and telephone operators answer more calls in the morning than in the afternoon. Late-day mental performance improves after napping, and the same may be true of muscle function.

Automobile accidents are more common at night because most people start to lose their ability to concentrate maximally after they have been awake for more than eight hours.

Another study from France shows that much of your early morning alertness can be restored just by napping or drinking coffee prior to driving (Annals of Internal Medicine, June 2006). Researchers tested young drivers during the day and again in the evening after drinking coffee containing 200 mg of caffeine, taking a 30-minute nap in the afternoon, or a placebo consisting of decaffeinated coffee. They drove 125 miles between 6:00 and 7:30 PM, and again between 2:00 and 3:30 AM. The doctors compared self-rated fatigue and sleepiness, inappropriate line crossings from video recordings, and polysomnographic recordings during the nap and subsequent sleep. Night time driving performance was similar to daytime performance for 75 percent of the participants after coffee, for 66 percent after napping, and for only 13 percent after the placebo. This study agrees with most other studies that show that napping or stimulants improve mental functions that require concentration or memory in the afternoon and evening.