Scientists –well, all of us- have been suspicious about the health risks of shift work for a long time now. Perhaps there is a reason buried somewhere in our genes that suggests night is for sleeping and daytime for working. Originally, no doubt, it was because it was difficult to see things in the dark and lighting, even when it became available, wasn’t very good.
But there is another reason: the Circadian Rhythm (from the Latin circa –around, and dies –day) which is often defined as physical, mental and behavioral changes that follow a roughly 24-hour cycle, responding primarily to light and darkness in an organism’s environment. The body clock, in other words. And there’s the clue: light and darkness. These are not just elements in our environment that we have come to expect, they actually have a biological meaning for us although this is, to a certain extent, entrainable. Malleable. As Wikipedia (sorry!) puts it: The rhythm can be reset by exposure to external stimuli (such as light and heat), a process called entrainment. The external stimulus used to entrain a rhythm is called the Zeitgeber, or “time giver”. But it can take a while to adjust –think of jetlag, or sleep disturbance after starting a new shift at a different time.
The body can adapt to many things, no doubt; the problems seem to arise when the pattern keeps changing. As folk wisdom attests, we are inherently creatures of habit –acquired behaviour patterns that are repeated so frequently they can become almost involuntary. As no less an observer of folkways than Samuel Johnson once said: “The chains of habit are too weak to be felt until they are too strong to be broken.” So one might ask why we –and many other animals- seem prone to develop these routines, these almost unconscious ceremonies. Is it simply a need for predictability? Or is it something deeper, something tied to our evolutionary past..?
In our evolutionary development we obviously experienced disruption of light/dark cycles –they occur as we travel through the seasons- but these are gradual and steadily progressive; shift work –especially rotational shift work- is not. And only recently has it become more obvious that there may be a price to pay. There have been several studies that have looked at this in various ways, but ‘Although epidemiological studies in shift workers and flight attendants have associated chronic circadian rhythm disturbance (CRD) with increased breast cancer risk, causal evidence for this association is lacking’ as the abstract of a paper published in Current Biology noted. I saw this in a July 2015 article in BBC News reporting on a study co-authored by Dr. Kirsten Van Dycke which suggested that the chronic need to re-entrain the circadian rhythm because of changing light/dark cycles can increase the risk for both obesity and breast cancer! http://www.bbc.com/news/health-33569161 Now, admittedly, the study was done on mice who were prone to develop breast cancers anyway, but when the light/dark cycles were switched over a long period of time (‘Mice prone to developing breast cancer had their body clock delayed by 12 hours every week for a year’) they developed them sooner.
Humans are obviously not mice, but it is difficult to control for possible contributing factors in the average human study: ‘Several scenarios have been proposed to contribute to the shift work-cancer connection: (1) internal desynchronization, (2) light at night (resulting in melatonin suppression), (3) sleep disruption, (4) lifestyle disturbances, and (5) decreased vitamin D levels due to lack of sunlight. The confounders inherent in human field studies are less problematic in animal studies, which are therefore a good approach to assess the causal relation between circadian disturbance and cancer.’ http://www.cell.com/current-biology/abstract/S0960-9822(15)00677-6
And the conclusion from this study? ‘Animals exposed to the weekly LD [light/dark] inversions showed a decrease in tumor suppression. In addition, these animals showed an increase in body weight. Importantly, this study provides the first experimental proof that CRD [Circadian Rhythm Disturbance] increases breast cancer development. Finally, our data suggest internal desynchronization and sleep disturbance as mechanisms linking shift work with cancer development and obesity’.
This is worrisome, to say the least. One could certainly argue that a woman with an increased risk for breast cancer –say a heditarily aquired BRCA1/2 mutation- would be best to avoid jobs involving chronic irregular body clock disturbance such as flight attendants, commercial pilots, and so on. But I’m not sure the risk is confined to that population. What about others –especially if they have additional life-style risks such as smoking, diabetes, alcohol issues?
And what about men? If –as the study suggests- a chronic body clock disruption may cause a decrease in tumour suppression, would that not suggest a similarly increased risk? The disruption also seems to have an additional risk for increased weight gain –obesity. Is the risk for type 2 diabetes therefore also increased? Clearly this is an area requiring much more research -further elucidation of the mechanisms involved and mitigation strategies at the very least. Sleep is so important –regular sleeping patterns…
I can’t help but remember the words of Shakespeare’s Macbeth talking to his wife after he has killed Duncan, the king:
Methought I heard a voice cry, “Sleep no more!
Macbeth does murder sleep”—the innocent sleep,
Sleep that knits up the raveled sleave of care,
The death of each day’s life, sore labor’s bath,
Balm of hurt minds, great nature’s second course,
Chief nourisher in life’s feast.
Art, once again, anticipating Science…