Light exposure at night suppresses the brain’s release of melatonin, a potent natural antioxidant and anticancer hormone. Previous studies have shown that even dim light exposure suppresses the release of melatonin. (Read my article on light exposure at night.)
In a landmark new study, researchers have discovered that when rats are implanted with breast cancer cells and exposed to either complete darkness at night or low-level, dim light at night, those in the complete nighttime darkness group had significantly slower breast cancer development and growth. This suggests that complete nighttime darkness suppresses cancer growth.
In a second part of the study, two groups of rats were started on the breast cancer drug Tamoxifen and either placed in a completely dark cage at night or a cage with dim light at night (similar in intensity to that of faint light coming in under a bedroom door), two remarkable findings were noted:
- When rats were exposed to dim light at night (which suppresses melatonin levels) the anticancer activity of Tamoxifen was inhibited. This finding suggests that light exposure at night inhibits the anticancer activity of Tamoxifen.
- When rats were exposed to complete darkness at night (which leads to normal, high nighttime melatonin levels) there was a dramatic reduction in tumor size and growth. This finding suggests that complete nighttime darkness makes the cancer cells more sensitive to the anticancer activity of Tamoxifen.
Normal daily fluctuation in melatonin levels in humans with a healthful sleep schedule (sleeping from 10pm-6am) with complete darkness at nighttime
In a third part of the study, to investigate if this anticancer effect was actually related to melatonin levels, the investigators gave the rats supplemental melatonin with or without Tamoxifen and exposed them to either nighttime darkness or dim nighttime light. They found the following:
- Melatonin supplementation by itself delayed the formation of tumors and significantly slowed their growth whether or not the rats had exposure to complete nighttime darkness or dim light at night.
- Tamoxifen caused a dramatic regression of tumors in rats with either high nighttime levels of melatonin during complete darkness or those receiving melatonin supplementation during dim light at night exposure…in other words, melatonin supplementation corrected the Tamoxifen resistance from light exposure at night.
The authors stated that “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.”
The Bottom Line:
In my opinion, there is no need to wait to wait for confirmatory studies to be done in humans before you start to implement a healthful sleep schedule (whether you have a history of cancer or not) as we already have a large amount of data from human studies suggesting that nightshift work, disrupted nighttime sleep and getting an inadequate amount of sleep (less than 6-7 hours) is associated with an increased risk of numerous cancers.
Cancer likes light exposure at night, so block at all light in your bedroom and get at least 6-7 hours of nighttime sleep.
Do your best to turn off all lights in your bedroom after 10pm as this is when your brain starts to release melatonin.
If you have history of breast cancer and you take Tamoxifen you need to get adequate sleep at night (and in complete darkness.)
If there is no way for you to avoid being exposed to light at night, blue light blocking filtered glasses and goggles can prevent melatonin suppression.
Do I advocate taking melatonin supplementation at night?
Numerous studies, in addition to this one, support the findings that this natural hormone is indeed anticancer. If you have trouble sleeping at night or are exposed to nighttime light (even dim light) you might consider taking a low-dose melatonin supplement. As always, I recommend you first discuss your interest in taking certain supplements with your cancer care team before you start taking them.
As more studies are done, there will likely other drugs that will be discovered to also be inhibited by light exposure at night.
The implications of studies like these are significant in my opinion for nightshift workers, people with problems sleeping, and people who are exposed to light from computer and TV screens at night.
Furthermore, not getting sleep at night and/or exposing yourself to light at night not only decreases the production of melatonin, but it also causes adrenal cortisol (stress hormone) to remain elevated at night when it is supposed to drop. Low nighttime cortisol is needed by the body to reduce systemic inflammation, help with normal bodily repair of tissue and cellular damage accumulated during the day, enhance immune activity, reduce insulin resistance and systemic production of free radicals. Persistently high levels of cortisol at night do the exact opposite and can lead to adrenal fatigue. (Read more about adrenal fatigue)
