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Why Sleeping in Darkness is Important to Your Health

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:

    1. During daytime

    2. During nighttime

    3. 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 essential 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 activity, and an unpolluted environment are also essential requirements for your best health.

If you have family members or friends who sleep with a light on (or have their children sleep with a light on), please consider sharing this information with them.


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I'll be more vigilant in my 'light's off!' routine with my 5 yr old - he doesn't like to sleep with the light off, but this study makes me think I should put my foot down, for a good reason. I found another article that links sleep, hormones and stress levels

I don't think it is as much of a risk for a 5 year old and the psychological aspect might not be worth the battle. What about the door cracked with the hall light on outside? or a nightlight?

When it comes to artificial light at night time not all kinds of light are equally harmful. The studies show that the warmer the light color temperature is (measured in Kelvin) the less it interferes with the body's ability to produce melatonin. The cooler the light is (the more over to the blue end of the spectrum) the more it's like daylight and the more it will inhibit melatonin production. Ideally we should all sleep in pitch darkness, but if your child absolutely must have a night light the best choice would be to get a light that is as dim and as warm as possible. I'd get a small light containing red LEDs. Red light is the warmest light in the visible light spectrum, so a dim red light would be the best choice for a night light.

I'm wasn't so sure about this one when I first read it. It seemed to be a modern day criteria but when I started to look at the history of how people slept, I'm tending to agree.
In former times before lights were invented, we all went to bed in the dark, usually when it got dark. Originally we had no windows even. Now, we stay up to all hours of the night and don't listen to our body clock which happens to be governed by the sun. I find it difficult in Canada because of the darkness during the winter..I feel the need to go to bed at 8:00 but I fight it. Who goes to bed at 8:00 anyway?? So it would explain why I feel so much better in the summer when I go to sleep the same time the sun does :-) Am I right?