Once upon a time there was no such thing as a human-made clock. Human beings relied on an internal biological clock, or a circadian rhythm, to inform the running of their day and night. Our ‘internal clocks’ are of course still relevant today, controlling such important functions as metabolism, physiology and behaviour, even though many of us are required to simultaneously run on a human-made clock.
We know that light is an important signal for our biological clocks, but interestingly, it seems so is food. Science has suggested there could be an independent gut-related internal clock. What’s more, there appears to be a link between disorders of circadian biology and disorders of the gastrointestinal tract, such as inflammation, and problems with propulsion of food matter.
How is it that our bodies ‘keep time?’ Apparently we have ‘clock genes.’ These genes help to tightly regulate metabolism, such as when we store energy or expend it, and recent research shows that diet affects our circadian rhythm and the expression of our clock genes. Factors such as nightly shift work and irregular food intake may cause functional problems in the digestive tract.
Interestingly, the appetite regulating substance, ghrelin, which is released by the stomach, appears to also be linked to circadian rhythms. Missing a single night’s sleep is enough to increase ghrelin levels and feelings of hunger. In addition, it appears that ghrelin may play a role during sleep to reduce appetite and reduce the urge to defecate.
The gut immune system also appears to have a circadian rhythm, with the gut immune cells at their most vigilant early in the day. This also seems to be the time that the stomach and intestinal cells peak in their proliferation. This could indicate that our gut defence is getting ready for the day. Interestingly, it could also work the other way, with the immune system influencing the circadian rhythm. There is some evidence to show that some inflammatory molecules can influence clock genes and may be a cause of fatigue.
References available upon request