In our busy lives with work, kids, and other responsibilities we often discover that doing things for ourselves, such as exercising, is put off to the end of the day when we actually have some time to devote to ourselves. The question this leads to is whether exercising in the evenings will affect our sleep cycles. It seems logical to assume that if you rev your body up at a time when it’s naturally starting to wind down, you may find it hard to get to sleep. But there are many factors that can affect how your body reacts. These factors include how much sleep you’ve been getting over the past weeks (in other words, are you sleep deprived or well rested); how hard and how long your exercise sessions are; and how late in the evening is your workout. Your personal response may vary. People respond differently to stimulus – some people can’t have any coffee or soda in the late afternoon or evening claiming that it will keep them up at night whereas others down Grande Mochas right before going to bed and claim no disrupted sleep whatsoever.
Sleep research is a relatively new science and there’s still much to learn. The body operates on a 24-hour clock, and every cell in every physiological system is guided by what are known as circadian rhythms. As the sun goes down, certain systems in the body (such as digestion) tend to slow down, while those processes that occur during the resting hours (such as cell repair) tend to ramp up. Certain triggers can throw off the body’s clock. Stimuli such as electric lights and TV can keep the body aroused and cause changes in circadian rhythms that may make it harder to get to sleep.
It appears that exercise can affect circadian rhythms, too. One study tracked changes in the levels of melatonin, a hormone that helps regulate the body’s sleeping and waking cycles, which normally peak at night. Published in the American Journal of Physiology–Regulatory, Integrative and Comparative Physiology, the 2003 study found that young adults (ages 20-32) and older adults (ages 55-73) both experienced delays in rising melatonin levels following late-night exercise. In this study, subjects performed three hours of light-to-moderate intensity cycling, and they didn’t begin the exercise until quite late at night—around 10:30 p.m. Subjects were then allowed to sleep for only six hours the next day. Subjects were monitored for only a three-day period, and the researchers did not track their ability to go to sleep the night after the study ended (when they were feeling the effects from having had only six hours of rest).
This raises a question: If they exercised again the following evening, would they be so tired that the subsequent exercise wouldn’t have much of an effect on their ability to fall asleep? As with many studies, there are still many unanswered questions, and it’s unclear what the longer-term effects might be from following a late-night exercise schedule. Of course, if you’re doing a 45-minute walk at 8 p.m., the effects may be much less noticeable (though everyone is different).
A considerable amount of research on jet lag and people who work night shifts has looked at the different triggers for disrupting and resetting a person’s body clock. A review of the research in a 2007 issue of the European Journal of Applied Physiology noted that exercise is associated with small phase delays in body rhythms. But the researchers noted that there was still much to learn about exactly how much exercise or what intensity of exercise might affect different people. There is some evidence that regular, moderate exercise may even help synchronize people, including those who travel between time zones or work night shifts. But this may also depend upon the time of day that the exercise is performed, and other variables may play a role, too.
From a practical perspective, getting adequate exercise and sufficient sleep are both vitally important to your health. If the only time you have to exercise is in the evening, and if you also find that you toss and turn in bed after a workout, then experiment with different times, durations and intensities of your workouts to see if you can find a regimen that doesn’t keep you up at night.