Preparing your body for medical treatment

When patients prepare for a major medical procedure — surgery, chemotherapy, a transplant — the focus is typically on logistics: fasting instructions, pre-operative labs, medication adjustments. Rarely does anyone consider the state of the patient’s circadian system. Yet a growing body of research suggests that how well your internal clocks are functioning before, during, and after treatment may meaningfully influence the outcome.

Your body responds differently at different times of day

Nearly every system involved in recovering from medical treatment — immune function, inflammation, tissue repair, drug metabolism, cardiovascular regulation — operates on a circadian rhythm. These are not subtle fluctuations. Heart attacks occur more frequently in the morning, likely driven by circadian surges in blood pressure and changes in clotting factors. Wound healing is measurably faster during the day. And the severity of cardiac injury during surgery has been shown to depend on when the operation is performed, with significantly lower risk in the afternoon compared to the morning.

This time-of-day variation extends to how the body handles the stress of treatment. Research has demonstrated that radiation therapy produces fewer side effects when administered at specific times — likely because healthy, non-tumor cells are more resistant to radiation damage at certain circadian phases. At least eight clinical trials have confirmed this pattern.

In oncology, one of the most compelling examples involves oxaliplatin, now a standard treatment for colorectal cancer. The drug was nearly abandoned during early clinical trials due to excessive toxicity. It was only when researchers tested it using circadian-timed infusion — with peak delivery at 4 p.m. — that safety was established and clinical efficacy was demonstrated. Chronomodulated delivery of a combination chemotherapy regimen reduced severe mucosal toxicity from 76% to just 14%, while increasing objective treatment response rates.

The hospital environment works against your clocks

Paradoxically, hospitals are among the worst environments for maintaining healthy circadian rhythms. Natural light is scarce. Sleep is constantly interrupted by alarms, procedures, and staff activity. Intravenous nutrition is often delivered around the clock, including during the hours when the digestive system is programmed to rest.

Research at Cincinnati Children’s Hospital found that bone marrow transplant patients who were fed intravenously 24 hours a day frequently developed hypertension that was difficult to control. When the team investigated the circadian implications, they realized that feeding during the biological rest phase — nighttime for most patients — was a likely contributor. A pilot trial restricting feeding to a 12-hour daytime window showed promising early results, with no ill effects and anecdotal evidence that patients returned to eating independently faster.

Hospital lighting compounds the problem. Most clinical settings maintain constant, dim artificial light that provides neither the strong daytime signal the brain needs to maintain robust circadian rhythms nor the darkness required for restorative sleep. Studies have shown that premature infants exposed to a 12-hour light/dark cycle are discharged an average of two weeks earlier than those kept in constant dim light or constant darkness. The implications extend far beyond neonatal care — to ICUs, oncology wards, post-surgical recovery rooms, and rehabilitation facilities.

Circadian disruption is itself a risk factor

The effects of circadian misalignment go beyond poor sleep. Even a single night of moderate light exposure during sleep has been shown to impair glucose regulation and cardiovascular function in otherwise healthy individuals. Shift workers — who experience chronic circadian disruption — face elevated risks of metabolic disease, cardiovascular events, and certain cancers.

For patients about to undergo major treatment, circadian disruption adds physiological stress to an already stressed system. A pooled analysis of over 1,000 cancer patients found that those with disrupted circadian rhythms had significantly worse survival rates and quality of life compared to patients with robust rhythms. The immune system, which is critical to recovery from surgery, transplants, and chemotherapy, is particularly sensitive to circadian alignment: immune cells follow a precise daily schedule of tissue surveillance during the day and systemic information-processing at night.

What patients and clinicians can do now

While personalized circadian medicine — matching treatment times to each individual’s internal clock — remains an emerging field, there are practical steps that can improve circadian alignment before and during medical treatment.

• Before treatment, patients can strengthen their circadian rhythms by maintaining consistent sleep-wake times, getting bright light exposure in the morning, avoiding light at night, and eating meals during daytime hours. These are not marginal lifestyle suggestions. They are interventions that directly influence the molecular clocks governing immune function, metabolism, and tissue repair — the very systems that will determine how well the body handles treatment and recovers from it.
• During hospitalization, simple environmental changes can make a meaningful difference. Dimming lights in the evening and at night, reducing unnecessary nighttime disruptions, restricting feeding to daytime hours where medically feasible, and covering or dimming blue-light-emitting devices in patient rooms all support circadian alignment. Some hospitals are beginning to install dynamic lighting systems that mimic the natural spectrum and intensity of daylight, allowing patients’ brains to maintain a coherent sense of day and night.
• For scheduling, where there is flexibility in when a procedure or treatment is administered, the emerging evidence supports considering time of day as a clinical variable — not just a logistical one. The research on cardiac surgery, radiation therapy, vaccination, and chemotherapy all point in the same direction: when matters.

The bigger picture

Precision medicine has made extraordinary advances in tailoring the what of treatment — the right drug, the right dose, the right molecular target for a given patient’s genetics and disease profile. Circadian medicine adds a complementary dimension: the when. It recognizes that the human body is not a static system waiting to receive treatment, but a dynamic one whose capacity to respond changes predictably over the course of every 24-hour cycle.

Preparing for medical treatment has always been about putting the body in the best possible position to heal. Circadian alignment is an increasingly evidence-based way to do that — and one that requires no new drugs, no new devices, and no additional cost. It requires only that we take the body’s own timing seriously.

References

• Montaigne et al. (2018) — Daytime variation of perioperative myocardial injury in cardiac surgery. The Lancet.
• Cederroth et al. (2019) — Medicine in the fourth dimension. Cell Metabolism.
• Ruben et al. (2019) — Dosing time matters. Science.
• Lévi et al. (1997) — Randomised multicentre trial of chronotherapy with oxaliplatin, fluorouracil, and folinic acid in metastatic colorectal cancer. The Lancet.
• Ballesta et al. (2017) — Systems Chronotherapeutics. Pharmacological Reviews.
• De Bree et al. (2020) — Non-specific effects of vaccines: current evidence and potential implications. Journal of Clinical Investigation.
• Zee et al. (2022) — Light exposure during sleep and cardiometabolic function. PNAS.