While most can agree that consistency is the most important factor when it comes to exercise, some might say that the time in which you do so is a close second. In fact, there’s even a term for it: chrono-exercise. According to nutritionist and personal trainer Mary Sabat, the idea, also known as chrono-biological or timed exercise, refers to the practice of engaging in physical activity at specific times of the day to optimize its benefits.
“The concept is based on the understanding that our bodies have natural biological rhythms that influence various physiological processes, including metabolism, hormone levels, body temperature, and cognitive function,” says Sabat. “These rhythms are regulated by an internal biological clock known as the circadian clock.”
And while every body is different in terms of needs and natural routine, a recent study (of more than 500,000 men and women between the ages of 40 and 69) pinpointed a key time of day as optimal for exercise: the 8 a.m. to 11 a.m. timeframe. Sabat expands on the science behind this finding, explaining that one key factor that affects the benefits of exercise at different times of the day is the body's hormone levels. For example, testosterone, a hormone important for muscle growth and repair, she says, tends to be higher in the morning. “This could contribute to improved muscle response and recovery when exercising during this time frame,” explains Sabat. “Similarly, cortisol, a hormone associated with stress response, typically peaks in the early morning and gradually decreases throughout the day. By exercising in the morning, when cortisol levels are higher, you can potentially harness the cortisol-induced energy boost to enhance your workout.”
Mike Julom, ACE certified personal trainer, CrossFit athlete, and founder of fitness platform ThisIsWhyImFit.com, offers a practical analogy, comparing the body to automobiles. “It's kind of like when you start your car on a cold morning; it runs a bit smoother once it's warmed up,” he says. “So, when your body is naturally warmed up, your muscles and joints are loose and ready to go, which means you're less likely to pull a muscle or get injured.”
The body’s internal clock plays a role here as well, says Julom, in that it’s often at its most alert and coordinated in the morning, making it an optimal time to engage in activities that require these skills. Plus, this period is typically when the heart rate and blood pressure are at their lowest, which might reduce the likelihood of overexertion and provide a safer environment for cardiovascular activities.
And then, there’s the matter of bone health. “Our bodies synthesize vitamin D most effectively in the morning hours,” says Julom. “So, if you're working out outside between 8 a.m. and 11 a.m., you're not just getting the benefits of exercise, but you're also supporting your body in effectively producing vitamin D, which is essential for bone health.”
Now, all that being said, most health and fitness experts will clarify that the time of day in which you exercise ultimately depends on your specific lifestyle and body’s circadian rhythm. “People's natural sleep-wake cycles, or chronotypes, differ significantly,” says Dr. Chester Wu, MD, who is double board certified in psychiatry and sleep medicine. “Some people are early risers, others are night owls, and most fall somewhere in between. These chronotypes can influence energy levels, performance, and preference for exercise at different times of the day.”
For example, perhaps you are more of a morning person who finds they have the most energy in the early hours of the day. Or maybe you’re more of a night owl, who gets their second wind in the evenings. “Research shows it's not just your motivation but your athletic performance that's likely to differ based on your chronotype, too,” exlains Dr. Wu. “Morning people are likely to perform better in the morning, night owls later in the evening.”
The type of exercise you engage in and the intensity in which you workout can impact timing as well. While testosterone and cortisol might typically peak in the morning, Dr. Wu argues that the body’s temperature is highest in the late afternoon, making that time optimal for high-intensity activities like HIIT or cardio workouts. Julom seconds this, noting the increased temp can provide enhanced flexibility and reduced injury risks.
Conversely, some low-intensity exercises like yoga or stretching can help calm and relax the mind and body, making them ideal for a time closer to bedtime to help you naturally wind down. “All sorts of things you might not think of as having a ‘best time’ like grip strength, hand steadiness, and balance all have their own best and worst times during the day based on your own circadian rhythm,” says Dr. Wu.
Age can also be a factor in your chronotype. Julom explains that younger people, especially teenagers, have a natural tendency toward being night owls, meaning they might feel more awake or energetic later in the day. “This is partly due to shifts in melatonin production, which affects sleep-wake patterns,” he says. “As a result, younger individuals might find evening workouts more appealing or effective.” But as one transitions to adulthood and middle age [30 to 50], your body’s circadian rhythm and life changes evolve and shift.
“The responsibilities of work, family, and societal commitments may also encourage earlier wake times and routines,” explains Julom. “So morning workouts might make more sense for this [middle] age group, aligning with their natural energy peaks. Then in the senior years, sleep patterns tend to change again. Many older adults experience advanced sleep phase syndrome, where they naturally feel sleepy earlier in the evening and wake up earlier in the morning. This means earlier morning workouts might align well with their energy levels. Additionally, for older adults, the body temperature rises earlier in the day compared to when they were younger, making morning exercises potentially safer in terms of injury prevention.”
And one should never discount personal preference. If you don’t enjoy or aren’t motivated to exercise at a certain time, you likely won’t stick to the routine. So, if your early evening run is your end-of-day reward for a hard day’s work, go with that. “Some people might feel more motivated and enjoy their workouts more at certain times of the day, regardless of the physiological factors at play,” says Dr. Wu. “This can lead to better adherence to a regular exercise routine, which is one of the most important factors for long-term health and fitness benefits.”
The medical professional goes on to explain that, despite the recent studies, more research is still needed to better understand chrono-exercise and the complex interplay of factors involved in individualized exercise timing. He concludes with sound advice: “The most important thing is to find a time that works best for you and stick to a regular exercise routine, as the benefits of regular physical activity are clear and well-established, regardless of when it's done.”