Recent breakthroughs in understanding the body's internal clock are transforming medicine, particularly cancer treatments. There is the time on your wrist or smartphone, and then there is the subtle, insistent rhythm that drives your body. In the 1980s, Israeli researchers uncovered the workings of this internal clock in the hypothalamus, a small brain gland. "This master clock responds to light through the suprachiasmatic nuclei (SCN) behind our eyes," explains Dr. François Duforez, a specialist in sports medicine and sleep. "This precise mechanism orchestrates metabolic activity on a roughly 24-hour circadian cycle, driven by day-night alternation."
It governs organ function, heart rate, hormone levels, body temperature, blood pressure, and sleep-wake cycles. Medicine has long harnessed light's influence on this system—light therapy, for instance, eases seasonal affective disorder (affecting 5% of French people), jet lag, and shift work disruptions. Now, chronobiology is fueling chronomedicine, tailoring treatments to patients' internal rhythms for optimal care.
Video of the day:The field gained momentum in 2017 when Jeffrey Hall, Michael Rosbash, and Michael Young won the Nobel Prize in Medicine for elucidating the molecular mechanisms of the circadian clock in cells. "Proving that every organ and cell has its own clock revolutionized chronobiology in medicine," says Dr. Sylvie Giacchetti, a hospital practitioner at APHP's Saint Louis Hospital Sénopole, speaking on France Culture. This accolade drew fresh talent to the discipline.
Chronotherapy research is now spotlighted, especially links between circadian disruptions and diseases like cancer. "Major studies show elevated breast cancer risk among night-shift nurses," notes Dr. Giacchetti. U.S. researchers found twice-daily light exposure benefits Parkinson's patients. Critically, timing oncology therapies by time of day cuts drug toxicity.
"The goal is to minimize side effects while boosting efficacy by optimizing drug timing—crucial for harsh anti-cancer regimens like chemotherapy," summarizes Dr. Annabelle Ballesta, INSERM researcher in personalized cancer chronotherapy at Paul Brousse Hospital, Villejuif. "The brain's suprachiasmatic nuclei set body-wide rhythms, syncing peripheral tissues like a central pacemaker. Every cell has a circadian clock."
Cancer cells, however, often lose this synchronization or lack a functional clock. "Chronotherapy exploits this mismatch," Dr. Ballesta explains. By administering chemo when healthy cells are least vulnerable—cancer cells, unrhythmic, remain sensitive—it reduces toxicity, allows higher doses, and improves results. "We target times when healthy cells resist best," she adds.
Chronotherapy is in active use at Paul Brousse Hospital. Optimal drug timing requires mapping rhythms in affected tissues, like tumors or the gut (prone to chemo-induced diarrhea).
"We analyze these organs and pathways to find low-damage windows, personalizing based on individual clocks—which vary by gender," says Dr. Ballesta. For irinotecan in colorectal and pancreatic cancers, the least toxic window is 5-9 a.m. for men and around 5 p.m. for women.
Benefits extend to anti-inflammatories for rheumatoid conditions like osteoarthritis—best taken at night for morning pain relief. In cardiology, Lille University Hospital found fewer post-heart surgery complications in afternoons (8% vs. 15% mornings). Patient chronotype matters: morning types have shorter clocks; evening types, longer.
Chronotype shifts rhythms, influencing optimal timing. Measuring them individually—via wearables tracking activity, temperature, heart rate—is key. "Next, we'll decode molecular mechanisms in cells through multidisciplinary in vitro and mathematical studies," Dr. Duforez notes. Reliable rhythm prediction will optimize drugs and surgeries. The potential is vast!
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