Does it matter when you work out?

Some people swear by morning workouts, while others hit the gym after the workday is done. We asked Michael Y. Mi, a cardiologist and Harvard Medical School instructor in medicine at Beth Israel Deaconess Medical Center, if the timing of a workout matters.

There are two aspects of optimization that are related — how to have a good single workout vs. how to have a better or faster adaptation over time. For adaptation, the biggest factor is not necessarily physiologic but psychologic; motivation and effort determine exercise load and intensity, which will drive pace of improvement. On the margin, one can consider other factors such as altitude — e.g., living at high altitude but training at a low altitude to improve aerobic capacity while maintaining training intensity — or higher temperature exposure to develop better heat tolerance.

But for a single session and long-term success, preparation is the key with sufficient sleep, fuel, hydration, and warmup. Various dietary approaches have been tied to manipulating metabolism, with a few demonstrating benefits like carbohydrate loading, creatine, and caffeine.

And while our metabolism does change throughout the day, if keeping to a time detracts from the core aspects of exercise — effort and consistency — it’s a downside.

That said, for people who follow a typical sleep schedule, our lowest resting metabolic rate tends to be in the late night and highest in the late afternoon, with a difference up to 10 percent. Interestingly, our body’s preference for carbohydrate or fat-burning also shifts throughout the day, with greater carbohydrate use in the early morning and lower in the evening and the opposite timing for fat.

So athletic performance tends to be better in the late afternoon when one’s body temperature is highest. Relatedly, it’s also important to warm up a cool body, such as in the morning, before heavy exertion. If exercising outdoors, one should be mindful of excessive heat or cold. There’s also some research to suggest vigorous exercise right before bedtime raises body temperature and may delay sleep onset, which would detract from recovery.

Different people may have different chronotypes and times when they feel the best, and it is not worth forcing a theoretically optimal time at the expense of consistency. Habit formation is a big factor, and having a routine helps overcome the psychological hurdle of making the deliberate choice to exercise. Building exercise into an established routine such as before or after work creates mental cues and makes adherence easier.

Exercise can boost energy and mood afterward, so morning sessions may help with the rest of the workday, and mornings may also offer fewer distractions and social obligations. People should ultimately pick a time that fits with their schedule for consistency.

—As told to Anna Lamb, Harvard Staff Writer