When Should We Exercise?

A morning workout can help with weight loss, but training later on also has health benefits, writes Gretchen Reynolds

- Gretchen Reynolds

Morning exercise has very different effects on metabolism than the same workout later in the day, according to an ambitious new animal study of exercise timing. The study, which involved healthy lab mice jogging on tiny treadmills, mapped hundreds of disparities in the numbers and activities of molecules and genes throughout the rodents’ bodies, depending on whether they ran first thing in the morning or deeper in the evening.

Many of these changes related to fat burning and other aspects of the animals’ metabolisms. Over time, such changes could substantially influence their disease risks and well-being. And though the study featured rodents, its findings likely have relevance for any of us who wonder if it is better to work out before work, or if we might get as much – or more – health benefit from after-hours exercise.

As anyone with a body knows, our internal operations and those of almost all living creatures follow a well-orchestrated and pervasive 24-hour circadian rhythm. Recent studies in animals and people show that almost every cell in our bodies contains a version of a molecular clock that coordinates with a broader, full-body timing system to direct most biological operations. Thanks to these internal clocks, our body temperature, blood sugar, blood pressure, hunger, heart rate, hormone levels, sleepiness, cell division, energy expenditure and many other processes surge and slow in repeated patterns throughout the day.

These internal rhythms, while predictable, are also malleable. Our internal clocks can recalibrate themselves, research shows, based on complex cues from inside and outside of us. In particular, they respond to light and dark but are likewise affected by our sleep habits and when we eat.