On January 31, it was reported that the COVID-19 pandemic has led to a delay of 2-3 hours in the average sleep time, with over 300 million people in China suffering from sleep disorders. But why do some people still feel groggy despite getting 8 hours of sleep? The answer lies in our circadian rhythms.
Circadian rhythms are natural, internal processes that follow a 24-hour cycle, regulating our sleep-wake patterns. This biological clock, often referred to as the "body clock," affects various physical and mental functions, helping to synchronize our body with the day-night cycle.
The phenomenon of the biological clock was first recognized through observations of rhythmic changes in plants and animals. The French astronomer Jean-Jacques d’Ortous de Mairan, in the 18th century, was among the first to use scientific methods to study circadian rhythms. While studying the Earth's movements, he noticed that the leaves of the sensitive plant (Mimosa pudica) would open during the day but close at night.
Although earlier studies had noticed biological rhythms in living beings, it was not clear whether humans had an internal biological clock responding to external stimuli, or if it was merely a behavioral observation.
In the mid-20th century, significant advancements in circadian rhythm research were made. Pioneers like Erwin Bunning, Jürgen Aschoff, and Colin Pittendrigh contributed to the study of biological clocks in plants, animals, and humans. Bunning focused on the rhythm of leaf movements in plants, Aschoff studied human body temperature and activity rhythms, and Pittendrigh studied the movement rhythms of fruit flies.
In 1971, Seymour Benzer and Ronald Konopka discovered a mutation in fruit flies that disrupted their normal 24-hour cycle, altering their activity patterns. This mutation affected a gene that controls circadian rhythms, which was later named period. Although fruit flies seem very different from humans, their circadian rhythm mechanisms share remarkable similarities with mammals, including humans.
In the following years, geneticists Jeffrey C. Hall, Michael Rosbash, and Michael W. Young successfully isolated the period gene and discovered that it produces a protein, PER, which fluctuates in a 24-hour cycle. The accumulation of PER protein during the night and its breakdown during the day helps explain how the biological clock works. This groundbreaking work earned them the Nobel Prize in Physiology or Medicine in 2017, with the Nobel committee recognizing their work for explaining how organisms align their biological rhythms with the Earth's rotation.
The importance of circadian rhythms in human health cannot be overstated. In October 2020, a team from the Fralin Biomedical Research Institute and Argentina’s National University published a study in Science Advances, revealing that chronic disruption of circadian rhythms could alter the tumor microenvironment, making it more favorable for cancer growth while hindering the immune system’s ability to fight cancer.
The study suggested that circadian rhythm disturbances could promote tumor growth by altering immune responses and cell cycle regulation. Specifically, a decrease in pro-inflammatory macrophages during disrupted circadian rhythms could help tumors escape immune surveillance and progress. This research is particularly relevant to people who experience frequent shifts in their sleep-wake cycles, such as shift workers or frequent travelers.
More recently, a team at the University of Rochester Medical Center published a study in Nature Communications highlighting the relationship between circadian rhythms and the brain’s waste clearance system. They found that the brain's lymphatic system, which clears waste during sleep, depends not only on sleep but also on the timing dictated by the body’s circadian rhythms. People who sleep during the day are at higher risk for neurodegenerative diseases, such as Alzheimer’s, due to disruption in this system.
Many studies have shown that disruption to circadian rhythms can lead to a range of health problems. However, a 2019 study published in Cell Reports challenged conventional thinking. Researchers at Northwestern University discovered that brief disruptions to the biological clock could actually protect neurons in a fruit fly model of Huntington’s disease. The study identified a gene controlled by the biological clock, hop, which, when removed, helped protect the brain from disease. This research suggests that a certain level of circadian rhythm "stress" could potentially slow or prevent the progression of neurodegenerative diseases.
Despite being counterintuitive, this finding opens up new possibilities for therapies that could protect the brain from such diseases.
Circadian rhythms are deeply ingrained in our biology, shaped by thousands of years of evolution. Unfortunately, modern life—characterized by shift work, artificial lighting, and irregular sleep patterns—often disrupts these natural rhythms, leading to health risks.
It is crucial for our health to align our lifestyle with our body’s natural circadian rhythms. Whenever possible, we should strive for a consistent sleep-wake schedule, with early bedtimes and wake times, to help our body function optimally and maintain overall well-being.
