Now that it’s getting dark before 5PM again, you might have an excuse not to hit the gym after work. New research suggests that exercising at night might give you a less optimal workout, according to a study by metabolism experts and professors at Northwestern Medicine. The scientists studied muscle tissues in mice, and found that the rodents had circadian clocks in their muscle tissue, which control metabolic responses and energy based on the time of day. The upshot is that muscle cells may be more efficient during normal waking hours.
When mice, which are nocturnal, exercised (yes, the scientists literally put them on treadmills), their muscles were more efficient at adapting to movement, using oxygen for energy, and processing fuel, like sugar and fat. At night, the mice’s muscles actually activated genes that helped them adapt to exercise. Since humans have these same genes, the researchers believe this means humans may also be able to exercise better during our natural, biologically programmed waking hours—i.e. when it’s light out.
As a result of our inbuilt circadian rhythms, which dictate our energy patterns during day and night, humans’ sleep-wake cycle is triggered by hormones that are released or blocked by exposure to light or dark. So, during summer, when the daylight is long, your muscles might be just fine if you go for an evening run—but in fall and winter, when the sun sets in the late afternoon, your body might start releasing melatonin (the hormone that helps you sleep) earlier. That may also let your muscles know that it’s sleepytime, unlike mice, whose muscles know that when it gets dark, their day is just beginning.
If you’re the kind of person who is fine with working out before or after work, or switch back and forth according to your schedule, just know that in the winter, you’ll probably have more energy for high-energy, demanding workouts in the morning—so get that intense SoulCycle or barre class out of the way, and save chill yoga sessions for evening.