Siri, When Should I Work Out?

Have you ever noticed how some people are able to wake up at 5 in the morning to work out, while others prefer midday or late night gym sessions? Researcher may have found clues as for the best time of the day to work out.

The mechanisms and function of sleep are still widely unknown. However, sleep has been shown to be controlled by circadian rhythms and homeostatic processes, as described in a lecture by Dr. Cavanaugh on circadian rhythms and sleep gating in Drosophila. The circadian mechanism regulates the timing of sleep and the homeostatic processes regulate both the amount and intensity of sleep. Circadian rhythms are modulated by internal clock mechanisms including different proteins that control the transcription of genes into proteins involved with circadian regulated behaviors, namely sleep-wake behaviors. Due to these internal mechanisms, humans can still produce circadian molecular products to regulate sleep-wake behaviors around 24 hours even when put into caves without day and night transitions. Those produced proteins act on the brain, the liver, and other parts of the body to control behavior. In humans, the internal clock is the suprachiasmatic nucleus.

Circadian rhythms can also anticipate and adapt to external environment. Circadian rhythms can become disrupted by exposure to light at night, the amount of sleep, and changes in sleep patterns. A good example of a disruption in circadian rhythms is jet lag. This occurs when a person moves to a different time zone, effectively shifting their sleep schedule and resulting in poorly timed circadian rhythms because their body is not used to the external environment.  Unfortunately, disrupting sleep cycles and circadian rhythms can lead to larger health consequences, such as certain types of cancers, mood disorders, obesity, and diabetes. The circadian internal clock mechanism also becomes less functional with age. 

Homeostatic mechanisms increase sleep pressure with the amount of time spent awake. To study sleep behavior in animals and humans, many researchers use EEGs. In the deepest part of sleep, the brain emits delta waves, which are waves with large amplitudes. According to Crocker and Seghal, increased power in the delta waves during deep sleep indicates higher homeostatic drive, showing researchers the level of sleep pressure on an animal.

In a study performed by Schroeder et. al., mice with a functional circadian clock systems and mice without a functioning internal clock were examined to study if exercise during a certain time of day can effectively regulate circadian rhythm or repair a broken clock. During their first experiment, healthy mice were either given access to run on their wheel anytime, given access only during the afternoon equivalent for mice since they are nocturnal animals, or given no time to run. They did not see much difference between the two running mice, but the running mice produced many more proteins in clock cells compared to the mice not given access to run. When testing mice without functioning internal clock mechanisms, after a few weeks of running, their circadian clock products were more likely to reach their targets, such as the heart, liver, and other regions of the brain, to control sleep-wake behaviors. Researchers saw that mice that ran earlier in the day produced less proteins in their internal clock cells than mice who ran later in the day, suggesting that the optimal time of the day to exercise is in the afternoon.

This theory fits well with ideas presented in the lecture given by Dr. Cavanaugh based on the competing sleep systems. The circadian and homeostatic systems are in competition with one another. As homeostatic begins to rise in afternoon, circadian mechanisms also rise to prevent premature sleep timing, counteracting the effects of homeostatic sleep drive. Schroeder et. al. concluded that exercising helps to regulate the production and function of internal clock mechanisms that regulates sleep-wake behavior, but researchers cannot definitively state when the best time to work out is. The researchers advise against going to the gym very late at night, but they do not distinguish between an early morning and an afternoon workout. 

References:

Cavanaugh, Daniel J., Abigail S. Vigderman, Terry Dean, David S. Garbe, and Amita Sehgal. "The Drosophila Circadian Clock Gates Sleep through Time-of-Day Dependent Modulation of Sleep-Promoting Neurons." Sleep 39.2 (2016): 345-56. Web. 28 Feb. 2017.

Crocker, A., and A. Sehgal. "Genetic Analysis of Sleep." Genes & Development 24.12 (2010): 1220-235. Web. 28 Feb. 2017.

Reynolds, Gretchen. "Why Afternoon May Be the Best Time to Exercise." The New York Times. The New York Times, 12 Dec. 2012. Web. 28 Feb. 2017.

Schroeder, Analyne M., Danny Truong, Dawn H. Loh, Maria C. Jordan, Kenneth P. Roos, and Christopher S. Colwell. "Voluntary Scheduled Exercise Alters Diurnal Rhythms of Behaviour, Physiology and Gene Expression in Wild-type and Vasoactive Intestinal Peptide-deficient Mice." The Journal of Physiology 590.23 (2012): 6213-226. PubMed. NCBI. Web. 28 Feb. 2017.