The early bird gets the worm -- that's the mantra the 5-a.m. gym-goers live by. However, exercise improves sleep quality, so why not work out later in the day, closer to bedtime? There's been a long-standing debate over the optimum time to work out, due to the physiological effects exercise has on the body. So, no matter what time you choose to work up a sweat, consider the pros and cons of a.m. vs. p.m. workouts.