When it comes to building strong circadian rhythms, physical activity is the red-headed stepchild of the zeitgebers. It’s easy enough to get outside during the day and block blue light at night, or restrict your eating to a certain part of the day. But by far, the zeitgeber people want nothing to do with is physical activity/exercise.

This is unfortunate because physical activity and exercise are both very powerful zeitgebers. I feel many in the circadian rhythms space do a disservice by stating that light is the most powerful cue.

Certainly light is the most overlooked zeitgeber, that is clear. But I liken the argument of which zeitgeber is most important to which wheel on your car is most important. Obviously, if you’re missing one, it’s that one.

There’s a robust amount of evidence that physical activity AND exercise are powerful zeitgebers. Much like feeding, it’s both an input and output of circadian rhythms. A review on the topic puts it nicely:

However, though less recognized, several studies have demonstrated that the non-photic cue, exercise, has similar effects on entrainment of the circadian clock and the sleep/wake cycle to that of photic light stimulus.

Source

So let’s take a look at why high levels of physical activity and exercise are so important

How exercise builds strong circadian rhythms

Exercise exerts it’s benefits on circadian rhythms through at least 3 independent mechanisms. This includes changes in metabolism, the output of circadian hormones, and changes in body temperature.

Exercise and changes in metabolism

The first way exercise impacts circadian rhythms is by altering cellular metabolism.Not only in muscle, but throughout the body. Exercise causes alterations in the sirtuin/NAD+ axis, which functions as the metabolic arm of the circadian clock.

Exercise chronically and acutely regulates sirtuin activity. In turn, sirtuin activity regulates the timing of cellular repair processes, epigenetic modifications within the cell, and mitochondrial dynamics.

Exercise changes the activity of circadian hormones

Secondly, exercise affects numerous circadian hormones such as cortisol, insulin, and leptin. Circadian variation in all 3 of these hormones helps synchronize circadian rhythms throughout the body.

Cortisol is an output of the master and adrenal peripheral clocks that acts to synchronize the master clock with the peripheral clocks. However, cortisol output is also controlled by exposure to stress and activation of the HPA axis. Thus, exercise induced activation of the HPA axis alters the daily rhythm of cortisol secretion.

Insulin, on the other hand, functions as the food entrainable oscillator. In order for food to have an effect on circadian rhythms, there must be a signal that identifies the feeding period. Recent evidence indicates insulin is that signal, increasing during the feeding period and decreasing during the fasting period.

Exercise helps maintain insulin sensitivity. In order for insulin to signal to cells, there needs to be insulin release from the pancreas as well as receptors on cells to respond to it. Exercise helps maintain the sensitivity of cells to insulin by increasing receptors to it.

Finally, leptin is a hormone that plays a role in telling us when to stop eating. When we eat, our fat cells signal that we’ve had enough by secreting the hormone leptin. Thus, leptin should also follow a circadian rhythm that signifies the feeding period much like insulin.

Unfortunately, leptin becomes dysregulated in obesity leading to leptin resistance, just like insulin. Leptin resistance has a negative impact on the circadian rhythm of feeding and metabolic activity. Exercise helps maintain leptin sensitivity in much the same way it helps with maintaining insulin sensitivity.

Exercise and core body temperature

When we’re physically active, we experience an increase in core body temperature. This may seem insignificant on the surface, but it’s actually a really important circadian signal.

Each one of our cells ticks to its own inherent rhythm. This may seem odd, but it allows flexibility. If all our clocks ticked at the same rate it would be more difficult to adapt to different conditions.

Fortunately, our environmental conditions help synchronize our cells together, like resetting a clock over and over again throughout the day. Core body temperature is a powerful signal that helps synchronize the clocks in our individual cells.

During exercise, our core body temperature increases. This activates heat shock proteins within cells and the activation of these proteins help set our clocks to the same time. Since the master clock plays a role in regulating core body temperature rhythm, temperature doesn’t have a direct effect on it.

However, core body temperature does alter the peripheral clocks through the activation of these heat shock proteins. Thus, core body temperature is an important cue to make sure all the clocks in all of our cells beat to the same rhythm.

Conclusion

Exercise and regular physical activity act as potent zeitgebers to help build strong circadian rhythms. Changes in metabolism, the output of circadian hormones, and changes in core body temperature are the primary ways that exercise alters circadian rhythms.

Though, it’s important to point out that timing of exercise is important. Additionally, when you exercise has a differential effect on shifting your circadian phase depending on your current chronotype. We’ll talk more about what’s going on with that in the next podcast.

Furthermore, your current chronotype is driven by both genetic and environmental factors and can change based on your exposure to zeitgebers. This includes light exposure, feeding rhythms, and social factors, and of course, exercise.