WebMD claims there is no evidence showing that calories are burned more efficiently at any specific time of day. Because time of day can influence people's emotions regarding exercise, however, it's important for each person to establish a habit of exercising at the same time every day.
The best time of day for exercise is governed by each person's circadian rhythms, family commitments and work schedules. According to WebMD, research shows that people who exercise in the morning are more faithful to their exercise commitment. Because body temperatures are lower in the morning, those who choose that time to exercise should take care to warm up sufficiently.