Living against our biological clock, working late-night shifts or eating at inappropriate times, can has been linked to health risks like metabolic syndrome, obesity, and diabetes in Current Biology - at least for mice.
Insulin action rises and falls according to a 24-hour, circadian rhythm, the researchers write. Mice unable to keep the biological time for one reason or another get stuck in an insulin-resistant and obesity-prone mode.
They took measurements of insulin in mice at different hours to reveal a regular pattern. Normal mice become insulin resistant during the day, when the nocturnal animals are mostly sleeping. Mice made unable to keep the time based on a genetic defect or constant exposure to light lost that rhythm. They also gained more weight when fed on high-fat mouse chow.
That responses to insulin would vary over the course of a day makes sense, even if it isn't the way scientists or doctors have often thought about it.
"From the work of Claude Bernard in the 19th century, the concept of homeostasis as the maintenance of a constant internal environment is deeply ingrained in our thinking about how organisms work," the researchers write.
It's also wrong, they argue, for the simple reason that an animal's environment follows an obvious daily rhythm too. Evolution will favor organisms that have an optimal response to that environment, which is rhythmic. And so it is that insulin action and blood sugar metabolism are tied to the time of day and to the internal mechanisms that keep track of that time. It's a challenging reality for us humans, living as we do today in the comfort of our homes, where the lights come on at the flip of a switch and the food is plentiful.
"Mediterranean diets in which the main meal is eaten in the middle of the day are probably healthier," says Carl Johnson of Vanderbilt University. It's probably best to eat a light supper and avoid snacking after dinner.
Diets that only limit when people eat surely won't reverse the epidemic of obesity, he says, but they would probably help.