While you might not get eight hours of rest at night, you sure can help your body optimize its rejuvenation process by turning off all lights during those few hours of sleep. Our biological clock resets every morning when exposed to daylight, but light at night, such as that from computers, televisions, digital alarms, and even cell phone alerts, disrupts the rhythm of the clock. Lack of rhythm can, at best, affect our performance at work or, at worst, leave us susceptible to mood disorders. While some of us dislike switching off our cell phones at night, complete darkness at night matters.
Researchers are debating the association between sleep and breast cancer, but they agree that light at night suppresses melatonin, a sleep-inducing neurohormone. They observed remarkable suppression in tumor growth during the melatonin-induced circadian rhythm (sleep-wake cycle) and attributed success to the immune-enhancing and antioxidative properties of melatonin. They found that light at night disrupts the activity of an enzyme called N-acetyltransferase (NAT) that converts serotonin, the happiness hormone, into melatonin.
We know that stress affects serotonin levels and robs us of our sleep, but so does light at night. So, why waste our efforts in staying healthy and happy just to give it up to light at night?
Light at night can trick the pineal gland into suppressing melatonin, without which our body cannot regulate estrogen levels. Increased levels of estrogen promote cell proliferation in hormone-dependent cells, such as those in breast tissues. Uncontrolled cell proliferation increases the risk of breast cancer, even in women who do not have mutant breast cells.
As melatonin is antiestrogenic, it prevents and helps treat breast cancer cells by regulating the effect of estrogen in the following ways:

• It binds with estrogen receptors in breast cells to block estrogen-induced cell proliferation.
• It modulates the activity of the aromatase enzyme that influences estrogen synthesis.

Suppressing melatonin seems to create a domino effect on our health because without complete darkness at night, melatonin levels are too low to effect change in estrogen levels and to dampen the subsequent proliferation of cancer cells.
Although researchers are yet to establish conclusive evidence that links sleep and breast cancer, it is one less excuse to pull an all-nighter. If anything, research agreement on melatonin is a good excuse to honor the body with complete darkness at night in the hope that the darkness hormone can ward off agents of cancer and revitalize the body for the break of dawn.

References:
Brennan, R., Jan, J. E., et al., (2006). Light, dark, and melatonin: Emerging evidence for the importance of melatonin in ocular physiology, 21: 901-08.
Clemons, M., and Goss, P., (2001). Estrogen and the risk of breast cancer, The New England Journal of Medicine, 344(4): 276-85.
Deguchi, T., and Axelrod, J., (1972). Control of circadian change of serotonin N-acetyltransferase activity in the pineal organ by the -adrenergic receptor, 69(9): 2547-50.
McClung, C. A., (2007). Circadian genes, rhythms, and the biology of mood disorders, Pharmacol Ther, 114(2): 222-32.
Sánchez-Barceló, E. J., Cos, S., et al., (2004). Melatonin–estrogen interactions in breast cancer, Journal of Pineal Research, 38: 217–22.