In bad news for regular jet-setters and shift workers, research by psychologists at the University of California, Berkeley, has shown that acute disruption of circadian rhythms can cause memory and learning problems long after people have returned to a regular schedule. While similar effects have been shown in jet-lagged subjects, the UC Berkeley study is the first to look at long-term effects and changes in brain anatomy after the subjects have recovered from jet lag.
The subjects in the study were female Syrian hamsters, chosen because their circadian rhythms are so precise. According to the researchers, they will produce eggs, or ovulate, every 96 hours to within a window of a few minutes. The hamsters were subjected to six-hour time shifts – the equivalent of a New York–to–Paris plane flight. The researchers then tested the hamsters’ performance on learning and memory tasks during the last two weeks of jet lag and a month after recovery from it.
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Learning difficultiesAs has been shown in previous studies, the hamsters suffering jet lag had trouble learning simple tasks that the hamsters in the control group had no problems with. What surprised the researchers was that these deficits persisted for a month after the hamsters returned to a regular day-night schedule.
More worryingly, the researchers discovered persistent changes in the brain, specifically within the hippocampus, a part of the brain that plays an intricate role in memory processing. Compared to the hamsters in the control group, the jet-lagged hamsters had only half the number of new neurons in the hippocampus following the month long exposure to jet lag.
"This is the first time anyone has done a controlled trial of the effects of jet lag on brain and memory function, and not only do we find that cognitive function is impaired during the jet lag, but we see an impact up to a month afterward," said Lance Kriegsfeld, UC Berkeley associate professor of psychology and a member of the Helen Wills Neuroscience Institute. "What this says is that, whether you are a flight attendant, medical resident, or rotating shift worker, repeated disruption of circadian rhythms is likely going to have a long-term impact on your cognitive behavior and function."
Circadian ryhthmsEach of us has an internal, 24-hour clock that drives our so-called circadian rhythm, which is reset every day by small amounts. When a person crosses several time zones in a short period of time, entering a time zone that is not synced with our internal clock, it takes much longer to reset this daily rhythm, resulting in jet lag until the internal clock gets re-synced.
This acute disruption of circadian rhythms can also cause general malaise as well as gastrointestinal problems because the body's hunger cycle is out of sync with meal times, Kriegsfeld said.
While for most travelers jet lag is a minor annoyance that is overcome within a few days, people who repeatedly cross time zones have been shown to suffer much more serious effects. Flight attendants and rotating shift workers have been found to have learning and memory problems, decreased reaction times, higher incidences of diabetes, heart disease, hypertension and cancer, and reduced fertility. The World Health Organization even lists shift work as a carcinogen.
Stress not the problemBecause jet lag can increase stress hormones like cortisol and disrupt reproduction, the researchers controlled for the effects of these by removing adrenal glands or ovaries in some of the hamsters and injecting normal levels of hormone supplements of corticosterone and estrogen, respectively. These hamsters showed a similar reduction in new, mature hippocampal neurons in the brain.
"The change was really dramatic and shows that the effect on behavior and the brain is direct, not a secondary effect of increased stress hormones," said graduate student Erin M. Gibson. "They are not due to increased cortisol concentrations."
The experiments also suggest that the low number of mature neurons in the hippocampus in jet-lagged hamsters was not due to decreased production of new cells, but rather, fewer new cells maturing into working cells, or perhaps new cells dying prematurely. Further studies are planned to determine the root cause of the reduction in mature neurons.