What time do you like to work out? For a lot of us, this varies from day to day, depending on factors like lifestyle, work, and other obligations. But have you ever thought of trying to time your workouts with your circadian rhythm? If you want to optimize your exercise efforts, this can be an easy and effective way of doing so.
In part one of this two-part article, we will explore what happens with our body’s circadian rhythm for the first half of the day so we can better understand our biology and how we can utilize this knowledge to time our workouts for maximum efficiency.
We’ll start at 6 am—the time our brain wakes up with a morning surge of cortisol—this is what turns our brain on at this time. VIP (vasoactive intestinal peptide) helps do this in long, light cycles, and VIP is highest at 6 am and lowest at 6 pm.
However, your circadian cycles can be dramatically different if you are overweight, specifically obese. For non-obese people, ghrelin (a prominent hunger hormone made in the stomach that has a half-life of one hour, and sends a signal directly to our pituitary gland and influences our metabolism) is high when cortisol is highest in the early morning. Ghrelin drops fast when food is eaten, but for the obese, ghrelin is actually much lower than expected in the morning. Once they eat, ghrelin will stay elevated for an extended amount of time (this is why obesity can be considered an inflammatory brain disorder that causes hormonal imbalance). This happens because of the inflammation associated with the higher leptin (the prominent satiety, fat-storage hormone that mainly primes you for reproductive fitness) levels in the morning in the obese.
At 6:45 am we can expect to see the sharpest rise in blood pressure in the entire day. This is due to many activated systems in the body getting us ready to fully supply blood to all vital areas to get us motivated to begin our day and search for food. This period of rapid blood pressure rise explains why we see so many cardiac deaths occur in early morning sleep or early wakefulness. This happens when cortisol is highest.
When morning sunlight hits your retina and receptors in skin cells throughout your body, the signal travels through the optic nerve to other regions of the brain, including the pineal gland. The light cue prompts melatonin levels to fall and serotonin and cortisol to spike within 30 minutes of waking. Adenosine levels decrease steadily as you sleep, and are low when you awaken, increasing alertness. The adenosine-cortisol-serotonin effect is most effective closest to dawn, another reason to try to rise with the light of day—if you have any morning rituals or a routine you like to practice early, like journaling, yoga, or stretching, do it outside! Merely watching the sunrise will have the same effect.
At 7:30 am, usually after an hour of light, melatonin is completely shut off in the brain. Then, at 8:30 am, the gut has been awakened and peristalsis (a type of intestinal motility) becomes more vigorous and bowel movements getting rid of yesterday’s food are very likely. This happens as protons flow to move serotonin sulfated (that means snuffed out, like a battery) by the light of the gut microbiome to get to the brainstem to become sulfated melatonin (serotonin converts to melatonin in the evening). Bowel movements are stimulated if food is eaten around this time as well. This is called the gastrocolic reflex. Cortisol, aldosterone, and ghrelin are all raised at this time to drive activity, increase our blood pressure and stimulate feeding. This is all yoked to AM sunlight stimulus, and it is blocked when we wear clothes or at work indoors in the AM.
Around 9-10 am, we have the highest secretions of the sex steroid hormones in humans and these pulsatile crescendos lead to our highest alertness at around 10 am in our day to allow us to explore our environment. Our ideal muscle coordination occurs at 2:30 pm, which adapts us best to hunt for dinner at this time. An hour later, at 3:30 pm we see our fastest reaction times develop from our motor systems in our CNS, making this a good time to get some movement or exercise in.
However, the day is hardly over at that time, and there still is an optimal time to exercise in the latter half of the day. Stay tuned for part two, in which we will take a tour of the circadian rhythm’s cycles throughout the afternoon and evening.