Sleeping in complete darkness at night could be key to successful breast cancer treatment, a recent study has revealed.
According to findings, being exposed to light at night makes breast cancer resistant to the widely used formonal therapy tamoxifen.
Such exposure shuts off night time production of the hormone melatonin, according to researchers at Tulane University in New Orleans, in the US.
Their study, published in the journal Cancer Research, suggests that this hormone is "vital" to the success of the drug in treating breast cancer.
The researchers examined the role of melatonin on the effectiveness of tamoxifen in fighting human breast cancer cells implanted in rats.
They found that melatonin by itself delayed the formation of tumours and significantly slowed their growth.
But tamoxifen caused "dramatic" regression of tumours in animals with either high night time levels of melatonin during complete darkness or those receiving melatonin supplementation during dim light at night exposure, the authors found.
They said that resistance to the drug was a growing problem among patients with hormone receptor-positive breast cancer.
"In the first phase of the study, we kept animals in a daily light/dark cycle of 12 hours of light followed by 12 hours of total darkness (melatonin is elevated during the dark phase) for several weeks," lead investigator Steven Hill, co-leader of Tulane's Circadian Cancer Biology Group, said.
"In the second study, we exposed them to the same daily light/dark cycle; however, during the 12 hour dark phase, animals were exposed to extremely dim light at night (melatonin levels are suppressed), roughly equivalent to faint light coming under a door."
He added: "Our data, although they were generated in rats, have potential implications for the large number of patients with breast cancer who are being treated with tamoxifen, because they suggest that night time exposure to light, even dim light, could cause their tumours to become resistant to the drug by suppressing melatonin production.
"Our study does not identify how much light exposure is needed to suppress night time melatonin production, and potentially drive tamoxifen resistance in humans, but we think that it could be as little as the amount of light that comes in the bedroom window from a street light.
"We are working toward conducting the studies that will answer this question."
Co-lead author David Blask, who also leads the Circadian Cancer Biology Group, added: "High melatonin levels at night put breast cancer cells to 'sleep' by turning off key growth mechanisms.
"These cells are vulnerable to tamoxifen.
"But when the lights are on and melatonin is suppressed, breast cancer cells 'wake up' and ignore tamoxifen."
Every year, around 41,500 women and 300 men in England are diagnosed with breast cancer.
Tamoxifen is an anti-oestrogen drug that is widely used to treat breast cancer.