While some people are up at the crack of dawn to lace up their running shoes, others can’t fathom a workout before noon. Finding the perfect time to exercise is as much about personal preference as it is physiology [1] [2]. Exercise is supposed to feel good — even if it’s simply the feeling of accomplishment afterwards. But if muscles are tight in the morning or working out too late disrupts sleep, exercise can feel counterproductive. And nobody wants a sad gym rat.

On the Clock — Why It Matters

New research suggests the body could adapt to regular gym dates, so if we hit the weight room every day at 4 pm, eventually we might perform better at that time than at any other time of day [3]. These findings are similar to earlier research, which suggests that sticking to a specific workout time can result in better performance, higher oxygen consumption, and lower perceived exhaustion [4]. But scheduling a workout is more complicated than choosing a favorite number on the clock and depends on a number of issues.

First, the body’s core temperature is an important factor in determining the quality of exercise. A cold body leaves muscles stiff, inefficient, and susceptible to sprains, whereas higher body temperatures leave muscles more flexible. Body temperature typically increases throughout the day, so muscle strength and endurance may peak in the late afternoon, when body temperature is highest  [5]. The afternoon is also when reaction time is quickest and heart rate and blood pressure are lowest, all of which combine to improve performance and reduce the overall likelihood of injury.

Hormone levels are also important in determining optimal workout time. Testosterone is important for muscle growth and strength, in ladies and gents. And the body produces more testosterone during late afternoon resistance training than it does during morning workouts [6]. Plus, the stress hormone cortisol, which aids in the storage of fat and reduction of muscle tissue, peaks in the morning and decreases throughout the day and during exercise [7]. But early birds, take heart: Morning workouts can be successful, too.

Pick A Time, Any Time — The Answer/Debate

Those who find it difficult to exercise past lunchtime can find solace in the fact that it's sometimes easier to keep a morning workout routine consistent. Afternoon and evening workouts are more likely to conflict with other responsibilities as the day progresses. Plus a full day’s work can take a serious toll on willpower — issues that can overcome any gym-goer’s best intentions. Morning workouts might also be a good option for stress-free snoozing. Since exercise increases heart rate and body temperature, working out too late in the evening (generally after 8 pm) can disrupt sleep and compromise the body’s ability to repair itself [8].

Research suggests the calorie burn during a workout depends less on the time of day and more on balancing physical activity with food intake. It's most important to find a realistic, consistent workout schedule. If working out in the morning is easiest, just make sure to spend a few extra minutes warming up muscles that might be extra cold and tight from sleep. And to keep afternoon workouts consistent, treat them as unbreakable appointments, find a workout buddy, and keep a gym bag in the car or office to minimize excuses. In the end, simply getting in a workout, no matter the time, is a step to living life at its best.

• The body tends to adapt to exercise at whatever time we usually hit the gym.
• Afternoon workouts might be best, since body temperature is higher and heart rate and blood pressure are lower then.
• On the other hand, some people find it easiest to stick to a morning workout routine.
• It’s most important to find a workout plan we can realistically maintain.

Are you an early-morning exerciser or do you prefer to work out later in the day? Tell us in the comments below!

Originally posted April 2011. Updated July 2012.