You should try: Late afternoon or early evening
You're naturally pumped up from 2 p.m. to 6 p.m. This is due partly to testosterone, which seems to be most responsive to exercise at this time of day, but also because your body temperature is at its peak. You should feel slightly stronger, more flexible (by as much as 20 percent) and your lungs should be working at optimal efficiency, boosting your endurance capacity for tough workouts. In a 2012 study of 20 Tunisian soccer players, the athletes performed best on tests of speed and endurance at 5 p.m., and other studies of experienced swimmers showed that they swam fastest in time trials that took place in the early evening.