The body has a network of daily (circadian) clocks that influences most areas of our health, including sleep and metabolism. A central clock in the brain is closely tied to daily sleep rhythms. But there are also clocks in other tissues - such as liver, muscle and fat - and they play an important role in metabolism.
Authors
- Jonathan Johnston
Professor of Chronobiology and Integrative Physiology, University of Surrey
- Alan Flanagan
Postdoctoral Research Fellow in Chrono-nutrition, University of Surrey
- Alex Johnstone
Personal Chair in Nutrition, The Rowett Institute, University of Aberdeen
Changing the relationship between our internal body clock system and daily changes in the outside environment is called "circadian desynchrony". In our lives, circadian desynchrony occurs in situations such as shift work and jet lag, where there is a mismatch between internal circadian rhythm and environmental rhythms, such as the light-dark cycle.
Past laboratory research of human circadian desynchrony has tested a 12-hour difference between our internal clock system and environmental changes, by inverting environmental and behaviour patterns over one to two days. The study showed changes in the participants' metabolism, with poorer blood sugar control. However, the effects of more modest circadian desynchrony, and recovery from these effects, are not well understood.
To address this, we conducted an experiment in which men and women were subjected to a five-hour change in their environmental and behavioural patterns. This was achieved by delaying bedtime by five hours. Measurements were taken before the five-hour delay and over the following five days.
All food was controlled and provided by the research team. The study participants were, on average, about 45 years old and overweight, but with no diagnosed health problems. We measured blood markers of metabolism, the brain clock's biomarker (melatonin), together with subjective feelings of sleepiness and alertness across the day.
We found there was an increase in evening sleepiness and a reduction in evening alertness immediately after the five-hour delay. This happened in parallel with altered rhythms of melatonin concentration, indicating a change in the brain clock. These changes gradually readjusted over the following five days, but without fully recovering to baseline.
The five-hour delay caused many changes in metabolism. These included reduced 24-hour energy expenditure in response to meals, slower release of stomach contents after breakfast, and differences in blood sugar and fat regulation.
In contrast to the sleepiness and brain-clock marker (melatonin), all the metabolic changes were fully readjusted in the five days following circadian desynchrony. Indeed, some of the metabolic changes had fully readjusted within only three days. The metabolic recovery, therefore, happened much faster than the recovery of the brain clock and subjective sleepiness.
What it means for people who do shift work or fly a lot
As with all research, there are limitations that should be considered. Human studies need to include participants who have very similar characteristics in order to reduce variability. It's possible that other groups of people - with different characteristics - may have responded differently. It is also possible that different results would be found with different time delays.
Still, our research confirms that circadian desynchrony impairs human metabolism but suggests that metabolic impairment is smaller and shorter-lived than changes to sleepiness and alertness. This finding is relevant to the large number of people around the world who do shift work or who fly a lot.
People subjected to circadian desynchrony should continue to focus on established advice to reset rhythms of sleepiness and alertness. This includes exposure to (or avoidance of) light at certain times of day. Taking a melatonin supplement at a time appropriate to the new behavioural schedule can also help.
In people who are generally healthy, it appears that the metabolic disturbance of circadian desynchrony will adapt relatively quickly. Nonetheless, it's important to have a good quality diet and, where possible, to minimise eating during the late evening and night .
Jonathan Johnston has received funding from BBSRC, MRC, ESRC and Diabetes-UK. He has collaborated with Nestle and has undertaken consultancy work for Kellogg's and International Flavors and Fragrances (IFF).
Alan Flanagan is the owner of and receives financial compensation from Alinea Nutrition, an education platform for healthcare professionals. He is also a volunteer member of Heart UK.
Alex Johnstone has a member of the British Nutrition Foundation Advisory Group. She receives funding from UKRI, the Scottish Government and UK charities.
