One study found that your body's ability to perform peaks in the afternoon. Your body temperature increases throughout the day, optimizing your muscle function and strength, enzyme activity, and endurance for performance. Between 2 p.m. and 6 p.m., your body temperature is at its highest.

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.

Morning workouts have an edge Working out in the morning—especially on an empty stomach—is the best way to burn stored fat, making it ideal for weight loss.

The gym with the most members is Planet Fitness, which has over 14 million members across its 2,000+ locations.