You’ve probably heard it before. It seems like everyone from part-time gym rats to professional fitness trainers is telling us we shouldn’t do it. “You shouldn’t exercise at night, because…”
The “because” might be any one of a dozen things, but the most common I hear are:
- “It releases endorphins.” (True, but that’s not a bad thing.)
- “It increases your adrenaline levels.” (Also true, but not a reason to not exercise at night as long as you don’t do it too close to bedtime.)
- “It raises your core temperature for several hours.” (True. Exercise does raise your core temperature for 5-6 hours afterward. Is that a reason not to exercise at night? Maybe, maybe not.)
- “You won’t sleep as well.” (Not necessarily true—the evidence is conflicting.)
What you won’t hear is the REAL reason you shouldn’t work out at night. It’s not about endorphins, or core temperature, or adrenaline. It’s not even about sleep. It’s about your body’s circadian rhythm—the internal clock that tells you when to sleep and to wake, when to be active and when it’s time to kick back. It turns out that muscles have their own circadian rhythm, and exercising at the wrong time of day—or night—means they just can’t work like they ought to.
Your internal clock does more than tell you when to sleep
Before we go any further, let me make a disclaimer: the research I’m going to talk about was…you guessed it…done in mice. I’ve made a big issue in the past of how research done in mice doesn’t necessarily translate to humans no matter what Big Pharma would like us to think. I’ve pointed out how many drugs we’ve seen that work great in mice but fail dismally when it comes to people.
This isn’t that kind of research.
For one thing, we’re not talking about animals that have been genetically engineered. We’re not talking about animals that have been “modified” to have something that looks like a human disease. We’re talking about looking at normal—both to mice and humans—biological processes and seeing what happens. And we’re not looking the brain or even the heart or lungs—we’re looking at the muscular system, which isn’t too dissimilar across the whole mammalian spectrum. So while it’s always good to take animal research with a grain of salt, in this case half a grain is probably all we need.
Also, you can easily test the conclusion on your own.
That said, here’s the science: Our bodies, like other animal bodies, have a built-in “clock” that’s attuned to the cycle of day/night, light/darkness. We call this the circadian rhythm. This internal clock is what tells us to get sleepier and sleepier the later it gets, then tells us when to wake. Different hormones fluctuate throughout the day and night according to our circadian clocks. Our brainwave patterns are influenced by our circadian rhythm. Our core body temperature rises and falls according to our circadian rhythm. It even plays a role in cell regeneration.
It’s a big deal. Disrupting your natural rhythm—with blue light from gadget screens, with shift work, or just with chronic low-level sleep deprivation—can lead to a variety of health problems from obesity and heart disease to depression.
You’ve may already know this. What you probably don’t know is that certain parts of our bodies, such as our lungs, livers, and spleens have their own internal clock that operates independently of our “master clock.” Now, research from Northwestern University in Chicago shows that muscles also have their own special clock, and that exercising at the right time of day might make a big difference in how much good it does.
Science says exercising at night won’t burn as much fat
Here’s the takeaway of the study: the same kind and same amount of exercise might have totally different results depending on what time of day you do it. To get a real benefit you need to exercise during what the human circadian clock says is normal waking hours—that is, while it’s daylight.
When the mice exercised during their normal active hours (nighttime, for mice—they’re nocturnal), their bodies did a much better job at turning on genes that allow adaptation to exercise. They used oxygen more efficiently. They recovered faster. And their bodies were better at burning glucose too.
Which leads me to the really interesting part: exercise during the daytime probably lowers your blood sugar more than nighttime workouts, and here’s why.
When you’re not exercising—when you’re resting or sleeping, for example—your muscles just burn the oxygen you’re breathing in for fuel. But when you do something more strenuous, the oxygen runs out in short order. The harder you exercise, the faster the muscles burn through oxygen. And when oxygen runs low, your body begins using sugar (glucose) for energy instead.
What the Northwestern researchers found is that if the muscles’ circadian clock was disrupted, they had a hard time burning glucose. Which means that ignoring your circadian clock when you choose your exercise time is going to make your body less effective at using glucose for muscle fuel. And that means more free glucose floating around...and thus higher blood sugar.
So what’s the bottom line? As always, any exercise is better than no exercise. If the only time you can fit a workout in is at 9 pm, by all means do it. BUT, if you want to get the most benefit, you’re better off doing it during daylight hours, preferably in the afternoon. Humans are designed to be active during the day and to sleep at night, with a peak in the afternoon. That’s when your muscles will be most efficient—you’ll get more bang for your buck and you’ll be less likely to suffer an injury.