Thursday, February 10, 2011

Effect of Light at Sleep on Health

I'm interested in two issues related to light at sleep: lower limit in intensity of light not affecting child's growth, and effect on risk of cancer. I didn't find an article directly discussing the relationship of light to secretion of growth hormone (GH). But since GH and melatonin secretions are related ("Melatonin stimulates growth hormone secretion through pathways other than the growth hormone-releasing hormone"), and the effect of light intensity on melatonin is regularly studied, we can focus on the latter. This letter, by Steven Lockley, a Harvard Medical School doctor, shows the change of melatonin suppression in relation to light intensity (see Fig 1(B)). You can see when the light intensity reaches 50 lux, the level of melatonin quickly starts to be suppressed until about 200 lux.[note] In plain English, if you need a better night sleep, and growth for the child, the light should be kept below 50 lux. According to Wikipedia, 50 lux is about the light intensity you get at your "Family living room". So I believe unless there's direct light shining on your eyelids, as those from your night lights or street lights, you should have a piece of mind. If you must have some light, such as that on your digital clock, make sure it's red, which is at almost the exact opposite of blue in visible light spectrum. Again according to Wikipedia, the blue light has the most detrimental effect on melatonin secretion. Now you know why those clocks are red.

Light at sleep is also known to increase the risk of cancer, the most studied type of which may be breast cancer. See Dr. Weil's short note, and this 2005 article.
[note] Other researchers did similar experiments, e.g., J.M. Zeitzer et al. in Am. J. Physiology in 2005. See its Fig 1(B). Their melatonin suppression onset started earlier at about 10 lux. But they had fewer data points.