Recent studies have suggested that our body’s circadian clock is an important factor in how the body responds to exercise. Our circadian clock or circadian rhythm is our body’s response to the light and dark cycle, and why most mammals spend their waking hours during the day and sleep during the night.
At least 50% of our metabolism is dependent on our circadian rhythms, and 50% of the metabolites (small molecules that play important roles in a healthy functioning metabolism) in our body work most efficiently depending on our circadian cycle (6). Therefore, It makes sense that exercise would be one of the things that’s impacted.
One study done at UC Irvine found that the exercise capacity of mice upon different exercise intensities and regimens found that overall exercise performance is substantially better (about 50% on average and more in some protocols) in the “mouse evening” (toward the end of their active time) compared to the morning hours. Mice are nocturnal, so their “waking hours” are during the evening and their “resting hours” during the day (3,4,5).
I know what you are thinking, “Karina, what is a mouse supposed to tell me about how my body responds best to training at certain times of day?” Mice are a genetic analog to the human genome and are extremely important for researchers to be able to move experiments into clinical trials done on humans. The reason most research is done on mice first is because we share on average 60-99% percent of identical genes with mice. You can learn more about why the mouse genome matters in research here.
So how does this further translate over to us? Well, another study done on 12 bipedal-diurnal-tailless humans showed a similar effect, having overall lower oxygen consumption while exercising in the evening compared to the morning, translating to overall better exercise efficiency (3,4).
The mechanism proposed from both studies have to do with the following metabolic pathways and metabolites:
ZMP (5-aminoimidazole-4-carboxamide ribonucleotide) showed to be higher in the “mouse evening”. Why do we like ZMP? ZMP is known to activate metabolic pathways that are related to glycolysis and fatty acid oxidation through activation of AMPK, which is a master cellular metabolic regulator. Therefore, it is likely to contribute to the increased exercise capacity in the evening (1).
AMPK is an enzyme that is activated in skeletal muscle during exercise. When activated, it stimulates energy-generating processes like glucose uptake and fatty acid oxidation and decreases energy-consuming processes like lipid synthesis (2).
Fun fact: ZMP is an endogenous analog of AICAR [aminoimidazole carboxamide riboside], a compound that some athletes use for doping (5).
From these findings we can infer that if you are a serious athlete, looking to maximize your training for strength and muscle growth, it is likely best for you to train in the evening. However, we can’t ignore the fact that some of us are just early birds, and some of us night owls. Circadian rhythm and exercise performance definitely has room for research to be done, and many more questions to be answered. However, these findings can definitely help point future researchers in a more specific direction!
-Karina Noboa, Assistant coach at Team LoCoFit
IG: @karinanoboa
Sources:
Ducker, G. S., & Rabinowitz, J. D. (2015). ZMP: a master regulator of one-carbon metabolism. Molecular cell, 57(2), 203-204.
Hardie, D. G., Ross, F. A., & Hawley, S. A. (2012). AMPK: a nutrient and energy sensor that maintains energy homeostasis. Nature reviews Molecular cell biology, 13(4), 251.
Sato, S., Basse, A. L., Schönke, M., Chen, S., Samad, M., Altıntaş, A., … & Treebak, J. T. (2019). Time of exercise specifies the impact on muscle metabolic pathways and systemic energy homeostasis. Cell metabolism.
Ezagouri, S., Zwighaft, Z., Sobel, J., Baillieul, S., Doutreleau, S., Ladeuix, B., … & Asher, G. (2019). Physiological and molecular dissection of daily variance in exercise capacity. Cell metabolism.
Cell Press (2019, April 18). Two studies explore whether time of day can affect the body’s response to exercise. https://www.eurekalert.org/pub_releases/2019-04/cp-tse041119.php
Bass, J., & Takahashi, J. S. (2010). Circadian integration of metabolism and energetics. Science, 330(6009), 1349-1354.
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