Do you have to wake up at the crack of dawn to get a good workout? Or should you be waiting until sunset to get your exercise in? What does it really mean to have a “good” workout? Optimal performance during the workout, optimized energy expenditure, and optimized adaptations can all be characteristic of a “good” workout. As can subjective outcomes: decreased stress, feeling “good” or “energized,” etc. The question remains: is there a physiologically-preferred time of day to exercise? Unfortunately, the research is scarce, however there are some bits of information that can be helpful in your decision-making. Research regarding time-of-day and exercise has mainly stemmed off of understanding circadian rhythms and hormone fluctuations throughout the day. I’ll spend some time explaining that before we get into the research findings and some other considerations.
Circadian rhythms are the daily cycles of physical, mental, and behavioral changes we undergo. All of our cells have synchronized “biological clocks,” which are responsible for regulating circadian rhythms.(1) There are four major hormones (chemical messengers) that are both controlled by circadian rhythms and associated with exercise: cortisol, testosterone, insulin, and glucagon.
Cortisol is known as the “stress hormone,” and yes, it definitely kicks in when we are under pressure. However, cortisol also works in less extreme situations, like (naturally) waking us up in the morning. Cortisol levels are typically high in the morning to draw us out of sleep, and then slowly decrease throughout the day. Relating to exercise, when cortisol gets into the blood stream, we see increased glucose (sugar) in the blood stream as well, which can supply muscles with energy to work.
Testosterone comes into play with muscle building through a process called anabolism. When testosterone gets into action, muscles and other tissues are being built up.(2) Testosterone levels are also positively associated with increased neuromuscular efficiency (i.e. brain to muscle connections work better and take less energy).(2) Men and women both have testosterone, and higher levels in men contribute to their ability to build muscle mass more quickly than women (fun fact!). In the world of circadian rhythms, testosterone is typically high in the morning, low in the late evening, and increases throughout the night while your body repairs.
Insulin is the glucose-storage hormone. It is most often released based on meal timing: eating puts glucose into the blood stream, and insulin comes out to store it in the liver or muscles if it is not being used. However, insulin does have a bit of a rhythm outside of eating. It starts off fairly high in the morning, peaks after meals and dips in between meals, and then gradually increases during typical sleep times.(3)
Glucagon is the opposite of insulin; it pulls glucose out of storage to be used for energy by cells and tissues. It has less of a rhythm, and more of a responsive role to increases in cortisol and exercise. As physiological stress increases, glucagon is activated to supply energy to the tissues that need it.
An additional factor controlled by circadian rhythms is core body temperature. During sleep hours, body temperature drops and then increases throughout the day, peaking in the early evening. A higher body temperature increases the speed of chemical reactions and nerve firing and could have indications for enhanced performance.(2)
Of the research that is out there on time of day and exercise, few have come to any definitive conclusions. Most of the studies I found have worked primarily with male subjects on specific exercise types, which can be limiting as far as applicability to the general public. That being said, I do think that there are some valuable nuggets in planning personal exercise. One final caveat, the research distinguishes between morning and evening exercise with no testing done during afternoon hours.
Let’s jump in: The first study I found looked at amateur male cyclists’ performance on a 1000-m time trial in the morning versus the evening. They measured the levels of various hormones in their blood before, immediately following, and 60 minutes after their tests for both sessions.(2) The hormones include those mentioned above and a couple more that I won’t go into detail about here. Here’s what they found: overall, performance was better in the evening sessions than in the morning sessions; i.e. participants had faster times in the evening. Hormonally, decreased levels of growth hormone and blood glucose were observed in the morning, along with higher insulin, cortisol, and testosterone levels.(2) This means that there was less glucose available to be used for exercise, despite testosterone levels aiding in efficiency. The authors concluded that performance was better in the evening due to the metabolic “milieu” being more ideal than in the morning.(2)
The other paper that I read looked at cortisol and testosterone, core body temperature, and other environmental and lifestyle factors that we will look at later on. From compiling data in other studies, the author concluded that cortisol and testosterone levels most likely do not have an impact of muscle/strength adaptations and may only play a minor role in exercise performance.(4) In regard to core body temperature, it is believed that higher temperature speeds up chemical reactions and may increase carbohydrate utilization, which facilitates muscle fiber mechanics. One study mentioned in the paper integrated a longer warm up period to elevate core body temperature in the morning and compared the morning exercise performance to evening performance with a normal warm up. The study showed that performance in the morning with the longer warm up was comparable to performance in the evening, when core body temperature is naturally higher.(4) So good news: you can achieve the elevated performance that is characteristic of evening workouts in the morning simply by extending your warm ups!
The second research paper that I read made a point to note environmental and lifestyle factors surrounding time of exercise. There is something called a “chronotype,” which is a time-of-day preference based on various physiological factors such as sleep-wake patterns, circadian rhythms, sleep, food intake, and maximal oxygen consumption during exercise.(4) Chronotypes are shaped by both genetics and habits, making each person unique in their preferred time for activity. If you consider yourself a “morning person” or a “night owl,” you probably have a good idea of what your chronotype may be.
Other factors that can explain exercise performance include eating habits throughout the day, being less flexible in the morning, grogginess upon waking, exercise time preference, recovery periods, and individual differences in exercise response and behavioral factors.(4) I’m beginning to understand why it is so challenging to uncover definitive research findings on time-of-day exercise: there are so many complex factors that differ person to person!
That being said, it is important to shape your exercise around YOU. If you have to be at work before 7AM and do not get out of bed until 6:30 most mornings, there is no use in planning a 4:30AM workout. Medical conditions and prescription drugs can impact your exercise timing as well. Be sure to talk with your doctor or pharmacist regarding physical activity and your condition or medication. Also, if you are involved in competition or performance events, it is helpful to exercise around the same time that you compete, so that your body’s physiological rhythms can adapt accordingly.(4) Your level of performance is influenced more by your preparation and motivation than by what part of your hormonal rhythm you are in. Recover well to perform well.
Wow, this was pretty dense! I’m glad you hung in there and hope you feel confident in the time of day you choose to exercise. Circadian rhythms are fascinating aspects of our physiology and play a role in our performance at various times of day, and at the end of the day (pun intended), when you exercise needs to work for YOU. Your preferences, schedule, recovery status, and goals will all play into what time of day is the best for you to exercise. The most important thing is that you ARE exercising. Work hard and recover well, and you will reap all of the benefits!