Abraham Haim, a professor of biology at Haifa University in Israel, said that the bluer light that compact flourescent lamps (CFLs) emitted closely mimiced daylight, disrupting the body's production of the hormone melatonin more than older-style filament bulbs, which cast a yellower light.
Melatonin, thought to protect against some breast and prostate cancers, is produced and secreted by the brain's pineal gland around the clock. Highest secretion levels are at night but light depresses production, even if one's eyes are shut.
A possible link between night time light exposure and breast cancer risk has been known for over a decade, since a study was published showing female shift workers were more likely to develop the disease. Prof Haim explained that a recent study by himself and fellow colleagues had found a much stronger association than previous research between night-time bedroom light levels and breast cancer rates.