Research links cancer risk with disruption of the circadian rhythm, the body’s roughly 24-hour system that coordinates sleep, hormones, body temperature, metabolism, and immune activity. The rhythm is synchronized mainly by the light–dark cycle, with the brain’s suprachiasmatic nucleus acting as a central clock.
The concern is especially relevant to persistent night-shift work, irregular schedules, and exposure to bright light at night. These conditions can disturb normal sleep timing and suppress nocturnal melatonin, a hormone involved in regulating the body clock and cellular processes. Researchers have also proposed effects involving hormone signaling, DNA repair, inflammation, metabolism, and immune surveillance.
The evidence should not be overstated: circadian disruption is associated with elevated risk for some cancers, particularly breast cancer, but it does not mean that one interrupted night of sleep causes cancer. A common mix-up is confusing circadian rhythm with sleep duration; they are related, but sleeping fewer hours and shifting the body clock are distinct exposures.