Cancer researchDec 3, 2005

Blood with low melatonin from premenopausal women exposed to light at night promotes growth of human breast tumors in rats

Circadian Biology Weekly Brief ↗PubMed ↗DOI ↗

Updated Apr 3, 2026

Abstract

Exposure to bright, white light at night can suppress melatonin levels and increase tumor growth in breast cancer models.

Increasing intensities of white fluorescent light during dark phases resulted in a dose-dependent reduction of nocturnal melatonin levels in rats.
Tumor growth and linoleic acid uptake were stimulated in rat hepatomas and human breast cancer xenografts exposed to higher light intensities.
Blood samples collected from healthy, premenopausal women showed that melatonin-rich blood collected at night reduced tumor proliferation when perfused to tumors.
Tumors perfused with blood from subjects exposed to bright light at night exhibited a pattern of high proliferative activity similar to daytime conditions.
These findings suggest that the circadian melatonin signal may play a role in inhibiting breast cancer growth and its effects can be disrupted by light exposure at night.

Simplified

The increased breast cancer risk in female night shift workers has been postulated to result from the suppression of pineal melatonin production by exposure to light at night. Exposure of rats bearing rat hepatomas or human breast cancer xenografts to increasing intensities of white fluorescent light during each 12-hour dark phase (0-345 microW/cm2) resulted in a dose-dependent suppression of nocturnal melatonin blood levels and a stimulation of tumor growth and linoleic acid uptake/metabolism to the mitogenic molecule 13-hydroxyoctadecadienoic acid. Venous blood samples were collected from healthy, premenopausal female volunteers during either the daytime, nighttime, or nighttime following 90 minutes of ocular bright, white fluorescent light exposure at 580 microW/cm2 (i.e., 2,800 lx). Compared with tumors perfused with daytime-collected melatonin-deficient blood, human breast cancer xenografts and rat hepatomas perfused in situ, with nocturnal, physiologically melatonin-rich blood collected during the night, exhibited markedly suppressed proliferative activity and linoleic acid uptake/metabolism. Tumors perfused with melatonin-deficient blood collected following ocular exposure to light at night exhibited the daytime pattern of high tumor proliferative activity. These results are the first to show that the tumor growth response to exposure to light during darkness is intensity dependent and that the human nocturnal, circadian melatonin signal not only inhibits human breast cancer growth but that this effect is extinguished by short-term ocular exposure to bright, white light at night. These mechanistic studies are the first to provide a rational biological explanation for the increased breast cancer risk in female night shift workers.

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Blood with low melatonin from premenopausal women exposed to light at night promotes growth of human breast tumors in rats

Abstract

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