There are several millions of shift workers in Germany. They bustle around in large factories, at the police, in printing companies or on guard duty. And they work in the healthcare system where physicians and nurses in hospitals are on guard to take care of patients with health problems just not willing to adjust to regular opening hours.
Researchers simulate shift work in the twilight of a lab
Nine-to-five-epidemiologists are interested in the alien species ‘shift worker’ for a long time now. They have made elaborate tables showing that shift work might go along with an increased risk for cardiovascular diseases. But because shift workers are not distributed equally in all social groupings this data had always been taken with a grain of salt. Now scientists at the Harvard Medical School are attending to the topic in an experimental manner and not in an epidemiological way. They are reporting in the Early Edition of the magazine Proceedings of the National Academy of Sciences (DOI 10.1073/pnas.0808180106). In order to observe live to a certain degree what happens metabolically to a shift worker they exposed 10 test persons to a simulated shift duty in the lab for 10 days. The participants experienced a total of seven 28-hr. days in a row with constantly dimmed light. That led to one complete twist of the circle of their sleep-wake-rhythm. The time they went to sleep was moved backwards for several hours every day until at the end of the protocol it reached again where it was before at about midnight.
The one who shifts becomes a metabolic
Contrary to other studies with similar questioning, the scientists in the team of Frank Scheer saw to a precise dietary intake: The test persons ate 4 isocaloric meals per 28-hr. day which provided an easier assessment especially of the blood glucose metabolism. And there’s something in that: The results of the Boston researchers are similarly depressing as the lab value teaching book about the metabolic syndrome. Particularly in those parts of the 10-day protocol where the day-night rhythm of the test persons was shifted for about 12 hours pretty much everything regarding blood values and other cardiovascular parameters was salient which you would consider cardiometabolically unhealthy. For example the level of the “slenderizer-hormone” leptin decreased for about a fifth of its normal. Despite an increased insulin secretion by a fifth as well, the postprandial glucose level was a significant six percent higher. The circadian rhythm in the cortisol secretion reversed. And the medium arterial pressure increased significantly. “Notably, circadian misalignment caused 3 of 8 subjects (with sufficient available data) to exhibit postprandial glucose responses in the range typical of a pre-diabetic state”, says Scheer. Before and afterwards it was not like that. According to the authors, these findings demonstrate the adverse cardiometabolic implications of circadian misalignment, as occurs acutely with jet lag and chronically with shift work.
Is leptin the root of all evil?
Since the collected data is very comprehensive it also allows hypotheses regarding the mechanism of how shift work upsets the cardiometabolic balance. The abnormally high cortisol level at the end of a wake period for example correlates with a higher insulin resistance and postprandial hyperglycemia. The pro-diabetic effects of shift work might be mediated via the hormonal stress pivot. On the other hand the considerably lower leptin level could lead by a stimulation of the appetite and a decrease of the basal metabolic rate to a diabetic metabolic situation and thus contribute to obesity in the long run. Perhaps all problems start out with the leptin produced in fat tissue. “We cannot explain satisfactorily the descent of the leptin with the variables normally influencing the leptin level”, says Scheer. The scientists therefore construct a new hypothesis that the disturbed sleep-wake rhythm decreases the leptin level rather immediately in yet unknown ways – here new researches will start.