When is the best time to exercise?

Analysis by Dr. Joseph Mercola Fact Checked

July 26, 2019

Story at-a-glance -

Among people who have maintained weight loss, 68% maintain a consistent workout schedule and 47.8% exercised first thing in the morning, suggesting consistent morning exercise may be helpful for weight management
If you’re in the habit of using time restricted eating, exercising before your first meal of the day will also allow you to take advantage of fasted exercise, which has a number of important metabolic benefits
Exercising while in a fasted state boosts fat shedding and maximizes the impact of AMPK, which not only forces the breakdown of fat and glycogen for energy but also plays an integral role in autophagy
Exercise and fasting together also yields acute oxidative stress, which benefits your muscle, and trigger production of BDNF, which helps rejuvenate your brain
Exercise can cause shifts in your circadian rhythm. The magnitude and direction of that shift depends on the time you exercise. Exercising at 7 a.m. or between 1 p.m. and 4 p.m. will shift your body clock to an earlier time. When exercising between the hours of 7 p.m. and 10 p.m., you shift your body clock to a later time

Exercise is a foundational pillar of optimal health and disease prevention, and something is always better than nothing. That said, there are many ways in which you can maximize your results.
High-intensity interval exercises, for example, are more effective than other types of exercise, which means you can get away with a lower time investment. The timing of your exercise could also make a difference.

Early morning exercise may aid weight loss

A study^1,² published in the July 2019 issue of the journal Obesity assessed the relationship between the timing of exercise and the ability to maintain weight loss.
The study included 375 participants from the National Weight Control Registry who engaged in moderate‐ to vigorous‐intensity physical activity (MVPA) at least two days a week, and who had successfully maintained their weight loss. At least 50% of their workout sessions occurred during the same time window, either morning, afternoon or evening.
Overall, 68% maintained a consistent workout schedule, and those who did also exercised more — 4.8 days per week compared to 4.4 days per week among those whose schedules were less consistent. The duration of their exercise was also longer.
The median duration for those with consistent schedules was 350 minutes per week, compared to 285 minutes per week among those with less consistent schedules. As a result, 86.3% of temporally consistent exercisers met the U.S. guidelines for exercise (150 minutes per week or more), whereas only 74.2% of less consistent participants met the guidelines.
Among those who kept a consistent workout schedule, 47.8% exercised first thing in the morning, suggesting the timing of their exercise might be a contributing factor to successful weight management.
That said, there were no significant differences in performance levels between the different time windows, highlighting that consistency is really the key issue. As noted by the authors:

“Greater automaticity and consistency in several cues were related to greater MVPA among all participants … Most participants reported consistent timing of MVPA.

Temporal consistency was associated with greater MVPA, regardless of the specific time of day of routine MVPA performance. Consistency in exercise timing and other cues might help explain characteristic high PA [physical activity] levels among successful [weight loss] maintainers.”

Reasons to exercise in the morning

There are many reasons to exercise first thing in the morning. For starters, doing it first means it’ll definitely get done, whereas afternoon or evening plans can easily get dashed by unexpected events or social invitations, or sheer fatigue and lack of motivation after a long day.
If you’re in the habit of using time restricted eating, exercising before your first meal of the day will also allow you to take advantage of fasted exercise, which has a number of metabolic benefits.
Exercising while in a fasted state essentially forces your body to shed fat,³ as your body's fat burning processes are controlled by your sympathetic nervous system (SNS), and your SNS is activated by exercise and lack of food.⁴
The combination of fasting and exercise also maximizes the impact adenosine monophosphate-activated protein kinase (AMPK),⁵ which not only forces the breakdown of fat and glycogen for energy but also plays an integral role in autophagy — the process by which your body cleans out damaged organelles and cellular parts.

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Benefits of fasted exercise

As noted in one 2012 study,⁶ “aerobic training in a fasted state lowers body weight and body fat percentage,” while “fed aerobic training decreases only body weight.” Exercise and fasting together also yields acute oxidative stress which, paradoxically, benefits your muscle. A 2015 study⁷ in the journal Biomolecules explains:

“Since the discovery of exercise-induced oxidative stress several decades ago, evidence has accumulated that ROS [reactive oxygen species] produced during exercise also have positive effects by influencing cellular processes that lead to increased expression of antioxidants.

These molecules are particularly elevated in regularly exercising muscle to prevent the negative effects of ROS by neutralizing the free radicals. In addition, ROS also seem to be involved in the exercise-induced adaptation of the muscle phenotype.”

In “The Exercise Mistake Which Makes You Age Faster,” Ori Hofmekler,⁸ fitness expert and author of several books, including “Unlock Your Muscle Gene” and “The 7 Principles of Stress,” addresses this issue as well, explaining that acute states of oxidative stress are:

" … essential for keeping your muscle machinery tuned. Technically, acute oxidative stress makes your muscle increasingly resilient to oxidative stress; it stimulates glutathione and SOD [superoxide dismutase, the first antioxidant mobilized by your cells for defense] production in your mitochondria along with increased muscular capacity to utilize energy, generate force and resist fatigue.

Simply put, exercise and fasting yield acute oxidative stress, which keeps your muscles' mitochondria, neuro-motors, and fibers intact. Hence, exercise and fasting help counteract all the main determinants of muscle aging.”

Hofmekler also points out that, combined, exercise and fasting “trigger a mechanism that recycles and rejuvenates your brain and muscle tissues." The mechanism he refers to is the triggering of genes and growth factors such as brain-derived neurotropic factor (BDNF) and myogenic regulatory factors (MRFs).
BDNF controls neurogenesis, signaling your brain stem cells to convert into new neurons,⁹ while MRFs are instrumental in muscle development and regeneration.¹⁰ In other words, fasted exercise may actually help keep your brain, neuro-motors and muscle fibers biologically young.

Fasted exercise boosts stem cell regeneration

Fasting also helps boost the generation of new stem cells — cells that can be used to heal and regenerate your tissues. This occurs during the refeeding phase, at which time your body starts rebuilding and replacing all those damaged cells that were cleared out during the fasting (autophagy) phase.
Regeneration can be further boosted by doing strength training the morning of the day when you're planning to break your fast. The reason for this is because during fasting, your growth hormone level skyrockets,^11,¹² rising as much as 300% for a five-day fast.¹³
That may sound paradoxical, since growth hormone typically rises in tandem with IGF-1, and IGF-1 inhibits autophagy. However, during fasting, the growth hormone receptors in your liver become relatively insensitive, so your IGF-1 level actually drops.
So, fasting can in some ways be likened to getting a growth hormone injection and a stem cell transplant, and by incorporating strength training at the right time, just before refeeding, you optimize all these regenerative benefits.

Fasted exercise improves insulin sensitivity

Fasted exercise is also a potent prevention strategy for Type 2 diabetes. In one 2010 study,¹⁴ those who exercised fasted increased their levels of GLUT4 — a muscle protein that plays a pivotal role in insulin sensitivity by transporting glucose into the cell — by 28%, compared to those who had a carb-rich meal before training, or controls (who did not train).
This despite eating a hypercaloric diet (receiving 30% more calories than required for health; half of which was from fat). According to the authors:

“This study for the first time shows that fasted training is more potent than fed training to facilitate adaptations in muscle and to improve whole-body glucose tolerance and insulin sensitivity during hyper-caloric fat-rich diet.”

Will evening exercise disrupt your sleep?

For all of the reasons discussed above, exercising first thing in the morning is likely an ideal choice. Exercise also tends to have an invigorating effect, which can propel you through the day when done early, and could backfire if done too late in the evening.
Since exercise increases in your adrenaline level, heart rate and body temperature, it could make it difficult to fall asleep. That said, evening exercise does work well for some people and, again, consistency is the most important component for success.
A study¹⁵ published in 2011, for example, found that when people exercised vigorously for 35 minutes right before bed they slept just as well as on nights when they didn't exercise. In fact, “The proportion of nonrapid eye movement sleep was greater after the exercise day than the control day,” the authors note.
Similarly, a poll¹⁶ by the National Sleep Foundation found that 83% of people said they slept better when they exercised (even within four hours of bedtime) than when they did not. Just 3% of late-day exercises said their sleep quality was worse when they exercised than when they did not.¹⁷
Interestingly, of those who exercised more than four hours before bedtime, 73% reported their daily routine allowed for adequate sleep, whereas only 65% of those who exercised within four hours of bedtime felt they had enough time in the day to get the sleep they needed.¹⁸
That said, the National Sleep Foundation concluded that exercise is good for sleep, regardless of the time of day it's performed, noting:¹⁹

"While some believe exercising near bedtime can adversely affect sleep and sleep quality, no major differences were found between the data for individuals who say they have done vigorous and/or moderate activity within four hours of bedtime compared to their counterparts (those who did vigorous or moderate activity more than four hours before bedtime) …

[T]he conclusion can be drawn that exercise, or physical activity in general, is generally good for sleep, regardless of the time of day the activity is performed."

Time of exercise influences your circadian rhythm

Indeed, there's a case to be made for exercise at virtually any time of day, including in the afternoon. Research^20,^21,²² published February 19, 2019, in the Journal of Physiology confirmed that exercise can cause shifts in your circadian rhythm, and that the magnitude and direction of that shift depends on the time you exercise.
Ninety-nine participants of varying ages performed one hour of moderate-intensity exercise on a treadmill for three consecutive days at one of eight time slots during the day and night: 1 a.m., 4 a.m., 7 a.m., 10 a.m., 1 p.m., 4 p.m., 7 p.m. or 10 p.m.
In summary, exercising at either 7 a.m., or between 1 p.m. and 4 p.m., will shift your body clock to an earlier time. As a result, you might find it easier to go to bed earlier when you’ve exercised at these times. Going to sleep earlier will also facilitate getting up earlier the next morning.
When exercising between the hours of 7 p.m. and 10 p.m., you shift your body clock to a later time, which might be helpful if you need peak performance later the following day (opposed to the morning). As reported by Neuroscience News:²³

“These findings suggest exercise could counter the effects of jet lag, shift work, and other disruptions to the body’s internal clock (e.g. military deployments) helping individuals adjust to shifted schedules.”

Interestingly, the most robust circadian advancements occurred when exercising in the afternoon (between 1 p.m. and 4 p.m.) — more so than exercising in the morning. As noted by the authors:²⁴

“These results are in contrast to conventional thinking that the circadian system is relatively unresponsive to afternoon zeitgebers [environmental cues that regulate your circadian rhythm], and thus could have numerous practical implications.

For example, the many individuals who are unable or unwilling to receive these zeitgebers in the morning could receive phase-advancing effects of exercise or outdoor light in the afternoon.

Conversely, for the goal of delaying the circadian system (e.g., for night shift-workers), it might be helpful to avoid afternoon exercise or bright light, which advanced the circadian system in the present study, as well as in previous studies.”

As you can see, you can find support for exercise at just about any time of day. The question is, what are you trying to achieve? Weight loss may be more easily maintained by working out first thing in the morning. Add in fasting, and you can boost things like stem cell regeneration and insulin sensitivity.
Meanwhile, afternoon or evening exercise may be helpful if you’re working shifts or traveling, by helping you reset your circadian clock. Knowing the variables involved, and how you can influence your health through the timing of your exercise, you can use it as a targeted tool to help you achieve your health aims.