What is the best time of day to exercise?

When it comes to exercise, the most important thing is that you do it. But if you like high-intensity exercise, research suggests that doing it in the evening might disrupt your sleep.

That’s because high intensity exercise can affect your internal body clock, known as your circadian rhythm.

“High-intensity exercise is anything that gets your heart rate up to a level where you’re only able to say a few words, but you can’t hold a conversation,” says Carol Harrison, a senior exercise physiologist at MD Anderson. “It’s sometimes called vigorous exercise, and it can include running, cycling, interval training and even strength training, if you’re really pushing yourself.”

These high-intensity exercises may change your circadian rhythm and delay the production of the sleep hormone melatonin.

“Research has found that if you do vigorous exercise in the evening, it delays the production of melatonin the next night, making it harder to fall asleep,” says Harrison. “What’s surprising is that the change happens so quickly, after only one night of exercise.”

The shift likely happens because your body adapts to support the evening activity. The exercise tells your body clock that you don’t want to be sleepy at that time, and it changes for you.

Your body relies on cycles

Melatonin production is one of several cycles in your body controlled by the circadian rhythm.

“The circadian rhythm is really fascinating,” says Harrison. “It also influences other hormones, like insulin for digestion and cortisol, which is related to stress. It is a big reason why having a consistent schedule is so important.”

When everything is in sync, your hormones start to flow at the times your body needs them most. Cortisol levels rise in the morning to get you going, followed by insulin to deal with digesting your first meal.

Then, melatonin kicks in in the evening to help you wind down to sleep.

“Research into how exercise affects our body clock is in its infancy; some others suggest the time of day you exercise affects weight loss and blood sugar,” says Harrison. “More research is needed, but it seems clear there is a connection.”

Evening exercise is not the only thing that can disrupt the circadian rhythm. If you eat at different times, or start sleeping later, it also has an impact.

“The most striking place we see it is with shift workers,” says Harrison. “There is evidence that people who work nights for many years are at higher risk for breast and prostate cancer, and the link may to be due to disruption of the circadian rhythm.” 

Irregular schedules because of shift work, or because of lifestyle choices, are also linked to obesity, which can increase your risk for cancer.

Low-impact exercise in the evening is best

That’s a lot of information about your circadian rhythm, but the most important thing to remember is that your schedule can affect your health.

When you plan your exercise, save high intensity workouts for earlier in the day and stick to lower intensity ones at night.

Studies have found that our circadian rhythm is not affected by moderate exercise, where you’re able to talk, but not sing.

“If you start to push that body clock back, and you end up not getting enough sleep, then you may disrupt many of your other cycles,” says Harrison. “Be aware of when you eat and sleep, especially if you work shifts. Keeping things regular can help you stay healthy and lower your risk for disease.”

Request an appointment at MD Anderson online or by calling 1-877-632-6789.