What time of day should I work out? The answer depends.

Many of us have run into the eternal workout quandary – what time of day should we exercise to reach our fitness and exercise goals? Is it in the morning before our daily work grind? After? Or during our lunch breaks?

Now, according to a study of swimmers across four Olympic games, swim times were the fastest in the evening, specifically for them, at 5:12 p.m. A separate study found that recreational cyclists saw faster trial times in the evening as well. 

Now, this has to do with our circadian rhythm – our body’s internal clock that syncs our sleep and eating time throughout the day. Our body clock takes cues from light exposure through the optic nerve and sends signals to other clocks in organs, muscles and fat tissues. These clocks can be influenced by external factors like meal times and activities, including exercise.

Now, performance aside, what about actual health benefits? When should we exercise to maximize those? 

"Everybody agrees that exercise is good, irrespective of time-of-day, but one can maybe fine-tune the metabolic outcomes of the exercise based on when you exercise," explains Juleen Zierath, an exercise physiologist at the Karolinska Institute in Sweden. From studying mice, though, she and her team found that they burned more fat when they exercised in the morning.

This is similar to another study that found one hour of weekly morning workouts – think resistance training, interval sprints, stretching and endurance – cuts ab fat and blood pressure in women. Doing the same routine in the evening, though, increases muscular performance. For men, evening sweat sessions lowered blood pressure and increased fat burn.

But, overall, the good news is that, based on another study, your body can adapt to whatever training routine and schedule you want. You just need to be consistent. Among the mice that were studied, consistency meant six weeks of training morning and afternoon before the mice reached equal endurance performance for both training sessions. 

"The simple notion here is that the clocks in our muscles are actually paying attention to when we train,” says Karyn Esser, a physiologist at the University of Florida. 

“Time your training bouts so that they are consistent with the time that you're going to have to be performing or competing at your peak." recommends Zierath.