The circadian clock, or circadian rhythm, is a 24-hour biological cycle deeply connected to our bodily functions. Many physiological phenomena—including body temperature, hormone secretion, blood pressure, and immune function—are regulated by this rhythm. This rhythm creates time periods when we are more susceptible to disease and times when treatment effectiveness is enhanced.Disease Onset and Time of DayMany diseases show peaks in onset during specific times of day. This is not coincidental but closely related to our body's rhythms.Heart attacks and strokes peak during the hours from early morning to mid-morning. A major factor is the sharp rise in blood pressure that accompanies waking, combined with increased secretion of cortisol, a stress hormone. While cortisol plays a role in switching the body to active mode, it also constricts blood vessels and promotes platelet aggregation. This makes blood clots more likely to form within blood vessels, increasing the risk of heart attacks and strokes. Additionally, blood pressure fluctuations place physical stress on the vascular endothelium, affecting the progression of arteriosclerosis. One study reported that approximately half of all heart attacks occur between 6 AM and noon.Asthma attacks frequently occur from late night to early morning. During this time, hormones that dilate the bronchi (cortisol and adrenaline) decrease in secretion, while acetylcholine, which constricts the bronchi, relatively increases. Furthermore, the parasympathetic nervous system becomes dominant at night, making airways more prone to narrowing. Additionally, the activity of eosinophils, a type of immune cell, becomes more active, making allergic reactions more likely and contributing to asthma attacks.Chronopharmacology: Timing of Treatment EffectivenessConsideration of the circadian clock is also important in disease treatment. Chronopharmacology, or chronotherapy, is a research field that optimizes dosing times so that drug effectiveness is maximized and side effects are minimized.Hypertension medications have been found to vary greatly in effectiveness depending on when they are taken. Many hypertension patients are "dippers," whose blood pressure decreases at night compared to daytime, but some are "non-dippers," whose blood pressure does not sufficiently decrease at night. Non-dippers are considered at higher risk for cardiovascular events. Taking specific antihypertensive drugs (such as ACE inhibitors or ARBs) at night improves nighttime blood pressure reduction and can lower cardiovascular event risk, as shown in numerous clinical studies.Antiallergic medications are also more effective when taken in alignment with when symptoms appear. Allergic rhinitis symptoms often worsen from nighttime to early morning. Taking antihistamines that may cause drowsiness before bedtime allows suppression of daytime symptoms the following day without interfering with sleep. This adjusts the timing so that blood drug concentration peaks during the day.Disruption of Circadian Rhythms and Health in Modern SocietyIn modern society, many people's circadian clocks tend to be disrupted by nighttime lighting, smartphones, and shift work. However, our circadian clock has been deeply encoded in our DNA through billions of years of evolution to adapt to Earth's 24-hour cycle. Therefore, even with some disruption to our daily rhythms, its basic function is maintained.This "misalignment" of the circadian clock is considered one of the root causes of modern health problems. For example, even when night shift workers sleep during the day and are active at night, their hormone secretion rhythms do not completely reverse. Unnatural states occur, such as decreased melatonin that should be secreted at night and shifts in the peak of cortisol secretion that should occur during active daytime hours. Numerous epidemiological studies suggest that this "circadian clock misalignment" increases the risk of chronic diseases such as diabetes, obesity, and cardiovascular disease.This indicates that circadian clock function remains effective in modern times; rather, the key to maintaining health lies in how to correct this misalignment and approach our natural rhythm. Modern medicine is advancing toward personalized medicine that incorporates the concept of the circadian clock, with development of treatment methods tailored to each patient's daily rhythm and circadian clock.Cancer Treatment and the Circadian ClockThe concept of the circadian clock is also emphasized in cancer treatment. Anticancer drugs damage both normal cells and cancer cells, but research is being conducted on dosing timing to minimize side effects on normal cells while efficiently affecting cancer cells.Cancer cells tend to proliferate actively during specific times of day, but the proliferation rhythm of normal cells is relatively constant in comparison. Utilizing this difference, administering anticancer drugs during times when normal cell activity is relatively calm may reduce side effects. For example, data exists showing that certain anticancer drugs cause reduced side effects such as nausea and fatigue when administered between 4 PM and 10 AM.Our Bodies Are Constantly ChangingOur bodies are not in a single static state but are constantly changing over the 24-hour day. This fluctuation significantly affects disease onset risk and treatment effectiveness. The concept of chronopharmacology, which considers the circadian clock to understand times when we are more susceptible to illness and to conduct more effective treatment, is becoming increasingly important. Personalized medicine that administers drugs at optimal times according to individual circadian clocks may change the future of healthcare. This connects not only to adjusting when to take medication but also to improving lifestyle habits and sleep patterns. In modern society where many people live night-oriented lifestyles, the importance of regulating the circadian clock should be recognized anew as fundamental to health management.SourcesThe Potential of Chronotherapy, Latest Trends in Time-Based Treatment, Disease Onset Time and Circadian Clock, Foundations and Clinical Applications of Chronopharmacology