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Why Sleeping in Darkness is Important to Your Health

Posted by ​By Dr. Ben Kim on July 31, 2008 (Originally published on on

Female night shift workers have a higher risk of developing breast cancer than the rest of the female population. An interesting study in the December 1, 2005 issue of Cancer Research provides new information that might partly explain why this is.

Researchers found that sleeping for several hours in nighttime darkness promotes a healthy blood level of a hormone called melatonin, which can significantly suppress the growth and proliferation of breast tumours.

They also found that sleeping while exposed to light at night causes a dramatic drop in blood melatonin levels, setting the stage for growth and proliferation of breast cancer cells.

This study provides evidence that may partly explain why breast cancer is five times more common in industrialized nations compared to less developed countries. It may also explain why visually challenged people have lower rates of cancer than people who have no problems with vision.

Here are the essential details from this study:

  • Researchers measured tumour growth and melatonin blood levels in rats that had malignant liver cancer or human breast cancer cells that were grafted into the rats
  • Tumour growth and melatonin blood levels were measured in response to introduction of blood from healthy, premenopausal female volunteers whose blood was taken:
  • During daytime
  • During nighttime
  • During nighttime following 90 minutes of exposure to white fluorescent light to their eyes
  • Tumours that were introduced to blood that was low in melatonin (samples taken during daytime and during nighttime following 90 minutes of light exposure) showed high proliferative activity
  • Tumours that were introduced to blood that was high in melatonin (samples taken during nighttime darkness) showed a significant suppression in proliferative activity

This brilliant study highlights the importance of getting restful sleep at night with minimal exposure to light. Doing so is esssential to promoting a healthy circadian cycle, which is needed to produce an optimal amount of melatonin, as well as other hormones that are beneficial to health such as growth hormone, testosterone, and erythropoietin.

Of course, getting restful night time sleep in darkness and having a high blood level of melatonin are not the only factors that influence your overall health status and risk for developing cancer. A clean diet, peace of mind, appropriate amounts of physical acitivty, and an unpolluted environment are also essential requirements for your best health.

If you know any people who sleep with the light on, especially children, please pass this article along to them or their parents.

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