Cancer Treatment Effectiveness May Depend on Time of Day, New Research Suggests

What

if the time you receive cancer treatment matters almost as much as what treatment you receive? A new study has suggested that the time of day cancer patients undergo immunotherapy could significantly affect how well the treatment works - a finding that is reshaping conversations in oncology and sparking global interest.

Morning or evening: What’s the best time for cancer treatment?

In a recent randomized study, scientists grouped patients with the same type of lung cancer and treated them with identical immunotherapy drugs designed to activate the immune system against cancer. The only variable here was the timing, as half the patients received their first treatments before 3 p.m., while the others were treated later in the day. The results were striking as those treated earlier experienced, on average, five additional months before their cancer progressed and lived nearly a year longer than those treated later. They were also significantly more likely to still be alive at the end of the study. Oncologists believe this kind of survival difference is a big win - especially when no new drug or technology has been introduced.

How does the body clock affect cancer care?

Scientists say the explanation lies in the body’s circadian rhythm - the internal biological clock that regulates sleep, hormones, blood pressure, metabolism, and immune function. It has been known for a long time that nearly every organ, and even individual cells, operate on timed cycles. Recent research shows the immune system is especially sensitive to timing. Immune cells rise and fall in activity throughout the day, which may influence how effectively they recognize and attack cancer cells. In cancer treatment, particularly immunotherapy, this timing could be critical.

Why does treatment timing matter?

The study mostly focused on PD-1 inhibitors, a class of immunotherapy drugs that prevent cancer cells from disabling immune “killer” T cells. These T cells naturally move in and out of tumours throughout the day, and according to evidence, they are most active in the morning. So, according to researchers, when immunotherapy is given earlier, more active T cells are already present in the tumour, allowing the drugs to trigger a stronger, longer-lasting immune attack. Blood tests from patients treated earlier supported this theory, showing higher levels of cancer-fighting immune cells. Interestingly, while the timing of the first dose appeared crucial, the timing of later doses seemed to matter less, a mystery that is still being investigated by the scientists.

Survival differences are striking

After following patients for more than two years, researchers found:

• Progression-free survival nearly doubled (11.3 months vs 5.7 months)
• Overall survival increased by almost one year
• Almost 45 per cent of early-treatment patients were still alive, compared with 15 per cent of late-treatment patients

These are outcomes that would typically be celebrated if achieved by a new breakthrough drug.

Doctors urge caution

Even though timing does matter, specialists are calling the findings “compelling” but urge caution. Immunotherapy drugs remain active in the body for weeks, raising valid questions about how a single timing difference could have such a dramatic impact. Because of this, multiple confirmatory trials are already underway, including studies in melanoma patients across major cancer centers. Researchers agree that replication across different populations and continents is essential before changing standard treatment schedules. If confirmed, this research could usher in a new era of chronotherapy — tailoring medical treatments to the body’s biological clock. Importantly, adjusting treatment timing could improve outcomes without increasing cost, toxicity, or drug dosage.