Sleep: the missing link?
How disturbed sleep may contribute to cancer risk
Posted Apr 01, 2010
When I launched the Anti-Cancer Challenge I had an intuitive sense that it should include sleep.
For one, my body has begun telling me that it isn't too happy with my irregular sleep patterns. It's not just that I feel a little tired during the day - that, I can live with. But I have noticed that I'm also more prone to colds and infections during prolonged periods of sleep deprivation - when working night after night to meet a deadline, say - than when I'm getting 7+ hours of sleep a night.
Moreover, I have observed that when I'm missing out on sleep frequently, my menstrual cycle shortens and I experience more noticeable PMS symptoms. At first I put this down to approaching menopause, but then I noticed that whenever I managed to sleep more regularly, my cycle would revert to its previous clockwork regularity.
Talking with the people around me, I gather that quite a few of us struggle to get restorative sleep. Prior to the industrial era, humans were exposed to bright, full-spectrum sunlight during the day and to complete darkness at night. Since the advent of electric light some 120 years ago, however, our exposure to natural light has been significantly replaced by artificial lighting. Moreover, westernized societies have become 24/7 communities where many of us are being exposed to artificial light during the night both at home and particularly in the workplace, thus disrupting our natural circadian rhythms.
Exposure to light at night interferes with melatonin, a hormone involved in a multitude of bodily processes including cardiovascular function, bone metabolism, hormone balance and cancer development and growth. Melatonin is produced in the pineal gland at night, when it's completely dark. When we are exposed to light at night, however, melatonin production is suppressed and this in turn may increase cancer risk though a variety of complex mechanisms.
Studies have shown that melatonin inhibits the development and growth of experimental models of breast cancer, whereas exposure to constant light stimulates mammary tumors in laboratory animals. Epidemiological studies back this up; they show that women working night shifts have a significantly elevated risk of breast, endometrial and colorectal cancer, while male night-shift workers are at a significantly increased risk of developing prostate cancer, presumably due to their increased exposure to light at night. (Read this excellent review for more details.)
Highlighting the seriousness of the problem, the World Health Organization's International Agency for Research on Cancer (IARC) concluded in 2007 that "shift work that involves circadian disruption is probably carcinogenic to humans."
A "Nourish"-reader helpfully pointed out in a comment to my previous post that nocturnal melatonin production is at least in part a function of how much daylight we are exposed to. A Japanese study found that spending one's day in dim indoor lighting leads to a weaker increase in melatonin levels at night than being exposed to bright outdoor sunlight. Thus not only shift-workers, but anyone leading an indoor and in-car lifestyle, like me, may be affected.
On the other hand, sleep duration - number of hours slept - is less clearly linked to cancer risk, and epidemiological investigations have yielded contradictory results. While a Finnish study indicated that women sleeping 9 hours a night or longer were 31% less likely to develop breast cancer than women reporting 7-8 hours of sleep per night, a British study found no significant difference in the breast cancer risk of those sleeping 9 hours or more and those getting less sleep. A third study actually detected a small increase in the breast cancer risk among women reporting more than 9 hours of sleep.
Conventional wisdom holds that 8 hours of sleep are "normally" required for optimal rest, but not all experts agree. "The fixation on sleep duration is unhelpful," comments Kevin Morgan, Professor of Gerontology at Loughborough University. "Sleep needs vary enormously from one person to another." As he sees it, the best criterion to measure sufficient sleep is "whether we feel restored by our sleep and are able to do what we need to do throughout the day." Age and gender also influence how much sleep we need, he says, making it impossible to set a "correct" number of hours people should sleep each night.
So where does all this leave people like me, who are not exactly shift workers but do spend a large portion of their nocturnal hours bathed in artificial light? David Blask of the Tulane University School of Medicine, New Orleans, speculates that non-shift working individuals who are not melatonin-deficient but who are chronically sleep-disturbed or deprived for other reasons may be at greater risk of developing cancer "due to the independent effects of immune suppression induced by sleep deprivation," though he notes that this hypothesis needs to be tested through more research.
What does this mean for my sleep challenge? I am still piecing together the puzzle, but initial conclusions are emerging. One is that sleep disruption and night-time work may have a negative effect on my immune system and overall hormone balance. In addition, daytime exposure to full-spectrum light is something I should increase from current low levels. (Spring and summer should help.)
Sleeping in a fully darkened bedroom should help stimulate melatonin production. And while sleep duration may not be directly relevant, I will try to increase mine a little for I do not currently meet Professor Morgan's criterion of feeling "restored and able to do what I need to do throughout the day."