It does not really matter. If anything, studies have shown that the best results (based on time of day) is the one in which you feel most comfortable. The happier you feel in the gym (rather than slogging through, tired or hungry or full from a meal or something) the better your results will likely be.

Strength, flexibility, and endurance fare better during the hours of 2pm-6pm when your body temperature is at its highest. Reaction times are also quicker during this window making it ideal for HIIT workouts. Afternoon or evening workouts may also help you to stay motivated, especially if you're not a morning person.

When you've been up and moving all day, your body may be better prepared to handle a sweat or strength-training session, physicians say. Plus, you have more fuel in your system from meals and snacks throughout the day to fuel your energy level and keep you going.

The largest gym company in the U.S. is Planet Fitness, which has over 2,000 locations nationwide.