The human body is finely tuned, especially when it comes to sleeping and being awake. The circadian rhythms are one reason why shift workers face the risk of various ill-health problems as they battle against the body’s natural internal clock. Circadian rhythms are thought to affect every cell in the human body and disruption has been implicated in obesity and cancer.
New research suggests caffeine consumption close to when the body is moving through a particular cycle in preparation for sleep re-sets the body clock back by one hour, which can disturb sleep.
The physiological reason is that caffeine delays the release and build-up of the hormone melatonin. This is the main sleep hormone released by the body to make us feel sleepy.
The research was carried out by the Medical Research Council’s Laboratory of Molecular Biology and University of Colorado. In a controlled study, researchers examined five people to see when melatonin starts to appear in saliva.
The study participants were held in a special facility for 49 days. In the room there were no clocks or any external light. This was so the people were unable to tell whether it was daytime or night time.
At various intervals the participants were given caffeine (equivalent of a double espresso) or a placebo three hours before they went to sleep. At the same time the people were exposed to dim or bright light (the bright light acted as a control as it also delays the human circadian clock). The scientists then studied when rise in melatonin levels occurred.
It was found that those who were given the caffeine saw their melatonin levels rise 40 minutes later than those given the placebo. The effects were later confirmed using human cells in a laboratory. This showed caffeine affecting adenosine receptors in cells, which suppressed the sleep hormone.
The findings are not simply confirmation of the influence of caffeine and restfulness. They can be used for further studies into how the body clock responds to 24 hour cycles and could help those with sleep disorders or who are overcoming jet lag or suffering with shift work.
The findings are published in the journal Science Translational Medicine. The paper is titled “Effects of caffeine on the human circadian clock in vivo and in vitro.”
