What Exactly Is the Ancient Circadian Rhythm?
Research on circadian rhythms has been ongoing for thousands of years. As early as the 4th century BC, a captain under Alexander the Great observed that tamarind tree leaves had different behaviors depending on the time of day—they opened during the day and closed at night.
In Traditional Chinese Medicine, it is believed that the flow of “Qi” in the body follows the rhythm of day and night. Each organ system operates on a 24-hour cycle, with its activity rising and falling in two-hour intervals.
Today, we know that the primary region responsible for circadian rhythm in the human body is the suprachiasmatic nucleus (SCN), a group of neurons in the hypothalamus. The SCN receives input from retinal cells and responds to visible blue light from the sun. In the absence of blue light, it triggers the release of melatonin—a hormone that induces sleep. In the morning, the SCN prompts the body to produce energizing substances that help us wake up and start the day.
The SCN acts like a conductor of an orchestra, coordinating attention, memory, physical performance, immunity, and sleep. And it's not the only "clock" in the body—major systems like the cardiovascular, metabolic, immune, and reproductive systems all have their own peripheral clocks that run on daily cycles. Even cells and microbes follow these rhythms.
When circadian rhythms fall out of sync—for example, due to staying up late—your organs want to rest while you're forcing them to keep working. This can lead to all kinds of health issues: poor sleep, indigestion, anxiety, reduced fertility, increased risk of injury, heart disease, stroke, and weakened immunity. Maintaining a healthy circadian rhythm is essential for overall well-being.
The Complicated Relationship Between Circadian Rhythms and Aging
Everyone wants a stable circadian rhythm, but that becomes more difficult with age. As we grow older, our circadian rhythm gradually weakens. One sign is that older adults tend to sleep less and wake up earlier, often with less energy than younger individuals.
According to research, aging affects circadian rhythms in two main ways:
1. Decline in Sympathetic Nervous Pathways
As we age, the link between the sympathetic nervous system and the biological clock weakens. The SCN receives fewer inputs from the peripheral clocks, making it harder for the body to adapt to changes in circadian rhythm. For example, a 50-year-old might feel exhausted for a week after one late night.
2. Decreased Light Recalibration
As we age, our eyes become less sensitive to light, and the strength of the light signals reaching the SCN weakens. Research shows that signal output within the SCN decreases with age, and the rhythmic release of neuropeptides and glucose metabolism also declines. Aging eyes let less light through, making it harder to reset the circadian clock.
Since the SCN relies heavily on light cues, older adults have a harder time adjusting their biological clocks than younger people. But that doesn't mean young people are safe. Irregular sleep schedules among today’s youth essentially simulate the effects of aging on circadian rhythms.
Older adults may have no choice but to avoid staying up late. But young people who constantly push their biological clocks to the limit with poor habits are effectively accelerating their own aging. Lack of sunlight exposure, staying up all night—these simulate age-related declines in light input, overwhelming the body’s natural rhythms.
Research has shown a close connection between circadian disruption and inflammation. Disrupting circadian rhythms promotes chronic inflammation and metabolic disorders. And chronic inflammation is one of the 12 hallmarks of aging. Staying in an inflamed state accelerates aging.
So, aging weakens circadian function—but poor habits in youth can do the same, leading to premature aging.
Want to Reset Your Circadian Rhythm? Start with a Good Breakfast
Is there nothing we can do about weakened circadian function? Actually, there is—one simple method is exposure to sunlight. As mentioned, the brain’s circadian center is highly responsive to light, so boosting light exposure can help realign your biological clock. In short: get outside during the day and keep your lights off at night.
Besides sunlight, researchers at Northwestern University have identified another way to help reset peripheral clocks—diet.
In a study simulating six hours of jet lag, participants were divided into three diet groups:
-
Meals scheduled according to the disrupted circadian rhythm
-
Meals aligned with the new time zone's daylight hours
-
Doubled breakfast portions while skipping dinner for three days, then returning to normal meals
Results:
-
Group 1 took 7–9 days to recover their rhythm
-
Group 2 shortened recovery time to 6 days
-
Group 3 (heavy breakfast, no dinner) recovered in just 5 days
This shows that dietary changes—especially emphasizing breakfast—can significantly shorten the time needed to correct circadian disruptions. A hearty breakfast can be especially effective.
Fighting against nature always comes with a cost. Staying up late and flipping your sleep schedule may seem harmless, but they burden your biological clock. If you wait until signs of aging show up, it may be too late to fully recover your youthful rhythm.
It’s time to return to a lifestyle of “rising with the sun and resting with its setting.” Eat well, sleep well, and protect your circadian rhythm—your youth depends on it.