AM vs PM: Does it matter what time you workout?

Whether you're a morning person or a night owl, you've probably wondered: is there an optimal time of day to work out? The debate between the benefits of an energizing AM sweat session versus an evening stress-relieving workout has raged on for ages. But the answer isn't as black and white as you might think.

Believe it or not, the time you choose to lace up your sneakers can impact everything from your mood to your metabolism. Surprising new research reveals how finely tuned your circadian clock is and how harnessing your body's natural 24-hour rhythms can help you burn more fat, build more muscle, and lower blood pressure when you work out.

Dr. Sue Decotiis, a medical weight management practitioner, advises her patients to exercise whenever possible. If it is more convenient for you to go to the gym in the morning, then you should definitely do so. It can make things easier for you. If you can only get to it in the afternoon, proceed accordingly.

Although the clock's significance may be limited, there are certain subtleties to take into account when exercising at various times.

According to Mauro Maietta, district fitness manager at Crunch Fitness, engaging in early morning exercise will enhance your metabolism and energy levels for the day while also aiding in setting up a regular regimen.  

Research supports the metabolism-boosting effects of an early workout.  The International Journal of Obesity, published a study in 2021, that observed a "strong linear association" between exercising and lowered obesity rates for participants who worked out in the morning compared to those who exercised in the afternoon or evening.

If you prefer to break a sweat after a long workday, afternoon workouts can provide a "stress reliever" and a way to decompress, says Maietta. Plus, body temperature and flexibility peak in the late afternoon, making it prime time for intense workouts. 

As for evening workouts, Maietta says they, too, can relieve accumulated daily stress and aid in unwinding. But be cautious wrapping up too close to bed, he warns, and allow ample time to cool down. 

"At least ensure you set aside 15 to 20 minutes for a substantial cool-down to bring your heart rate close to resting levels," Maietta advises evening exercisers. 

While men and women can both reap benefits from workouts no matter the hour, research suggests some differing outcomes based on sex. A 2022 study in the National Library of Medicine indicates women who exercised in the morning lost more fat and lowered blood pressure to a greater degree. Men benefitted more from evening workouts, showing enhanced mood, fat burning, and blood pressure reductions compared to morning sessions.

Both Maietta and Dr. Jon LaPook, chief medical correspondent for CBS News, emphasize that consistency with your workout routine is far more important than the specific time of the day you choose to exercise. LaPook even suggests integrating movement into your regular routine. Take the stairs rather than the elevator. Walk farther to pick up lunch or those last few blocks of your commute.   

"Make it so that you're not saying, 'OK, now I have to do exercise' — exercise becomes just part and parcel of how you live your life," says Dr. LaPook.

Even short 20-second to 2-minute micro-workouts squeezed into your day can add up over time. The key is not sweating over finding a perfect hour-long workout block. Instead, focus on integrating movement however possible.  

Listen to your body, Maietta advises. Adjust your workout schedule as needed. And, if you are trying to address specific fitness goals or health conditions, consult a professional for tailored guidance. 

Most studies into workout timing focus on the overall health effects rather than recommendations for specific fitness objectives like muscle gain, weight loss, or injury recovery. However, exploring some of the science behind both morning and evening workouts can help you choose what works best for your body and lifestyle.

According to a 2021 research in the International Journal of Obesity, 45 minutes of morning exercise torched 20 percent more calories over the following 24 hours compared to working out later in the day. This extra metabolic boost was especially apparent in women.  

The reason comes down to body temperature's impact on metabolism, explains exercise physiologist Dr. Jessica Hill.  

"Your (core body) temperature is increased from the exercise routine, resulting in an increased metabolic rate and additional energy burned after exercise," says Dr. Hill. She equates the post-workout calorie burn to revving a car engine hotter before shutting off the gas.  

So by exercising in the cooler hours, like early morning, before both ambient and body temperatures rise, you rev your metabolic engine hotter. The bodily afterburn sustains even after you towel off.

That same 2021 study on morning workouts reducing obesity risk also found that participants had improved blood glucose and insulin level regulation. Preventing blood sugar spikes and plummets helps manage conditions like diabetes or prediabetes. It also curbs cravings and overeating later in the day.

Per the 2022 study in the National Library of Medicine, women reduced resting blood pressure most effectively from morning workouts compared to other times of the day. Since blood pressure rises over the course of the day, exercising early in the morning keeps those levels lower following a workout as opposed to an evening sweat session.

Why? This blood pressure regulating effect is likely linked back to body temperature's role, too. 

"As your body temperature rises during exercise performed in cold conditions (early in the day), there's increased demand on the cardiovascular system," explains sports scientist Dr. Emily Weber. "Over time, this leads to positive adaptations like lowering resting blood pressure."

So by working out when both outdoor and internal body temps run cooler in the morning, your heart and circulatory system have to work harder to deliver warm blood. That added workload eventually strengthens over time, resulting in more significant blood pressure improvements than temperate or hot condition workouts later in the day.   

While exercise generally lifts mood whenever you do it, men seem to benefit more emotionally from evening workouts, according to research. No concrete data explains why as of yet.

However, the mental health boosts of breaking a sweat early are likely linked back to morning workouts that better regulate blood sugar and body temperature. Preventing blood sugar crashes preempts mood swings and crankiness. And running hotter internally may correlate with feeling happier externally, too.  

Some smaller studies also indicate morning exercisers feel more positive effects versus non-exercisers for the rest of the day. The exercise-induced surge of mood-enhancing hormones like endorphins and serotonin appears to linger longer when triggered earlier. But more research needs to confirm if timing plays a factor here or simply exercising in general produces these lingering positive emotions.

While larks lean towards mornings, night owls too can harness these potential benefits by working out in the evening hours:

The 2022 study on the differential impacts of morning versus evening exercise found men building more muscle mass by working out at night. Parallel research observes exercisers display higher strength and power output in the early evening hours as well.

Why do muscles perform better at night? Thank our circadian clocks. 

"Due to circadian rhythm, your strength, power, and tolerance to high-intensity exercise increases from morning until early evening as muscle temperature and nerve conduction velocity increase," says Dr. Weber.

Take advantage by scheduling weight training, CrossFit style metcons, or sprint sessions during peak evening hours to max out your potential. Just be sure to program in proper warm-ups and gradual cool-downs to avoid injury, especially if crossing over into late hours closer to sleep.

While the study on exercise timing and body composition found morning workouts better for reducing fat in women, men actually lost more body fat working out at 7 p.m. versus 7 a.m.  

This fat-burning boon for male night owls possibly relates to strength and muscle-building advantages in the evening. More muscle mass means a higher calorie burn 24/7.   

But regardless of gender, taking advantage of the circadian rise in body temperature later in the day by doing higher intensity training can spike calorie burn more post-exercise. That means potentially greater fat loss via a larger "afterburn" or excess post-exercise oxygen consumption (EPOC).

Maietta notes evening workouts can function as effective stress relievers after a long day. Exercise releases coping chemicals like serotonin and dopamine to blunt daily tensions. There's also evidence that day sweats may reduce the stress hormone cortisol more than morning workouts.

Transitioning from a high-energy workout to a lower-intensity cool-down also helps guide the body into a relaxed state for a better night's sleep.

While the pros and cons above might nudge morning versus night exercise decisions one way or another, optimizing your Zen comes down to mindfulness. What works best for your body, energy levels, and schedule trumps any rigid rule. 

"It's important to choose a time of the day that allows you to be consistent with your exercise routine. Consistency is often more critical than the specific time of day you work out," urges Maietta. 

So, take notes on your own limits and peaks. If you know realistically, you'll never pry yourself out of bed at 6 a.m. for pre-work sweat sessions, then don't force it. But if you see better consistency and effort by locking in sessions first thing before procrastination sets in, embrace the early bird schedule.   

You might also find your perfect medium by balancing morning and evening workouts based on what each uniquely targets better. Or mix and match for variety if one set schedule feels boring. 

For those short on time, remember micro workouts allow you to accumulate movement in tiny bursts spread over the course of a day. Even a couple minutes here and there still provides cardiovascular and strength training stimulus. 

The most important point is increasing your weekly exercise consistency by whatever scheduling, length, or training techniques keep you moving. Everything else — faster fat burn, better blood pressure, increased endorphins — comes along for the ride when you commit over the long run.