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).