Now, researchers have new evidence to explain why it matters not just what people eat, but also when they eat it.
Insulin action rises and falls according to a 24-hour, circadian rhythm, the researchers found.
What's more, mice unable to keep the time for one reason or another get stuck in an insulin-resistant and obesity-prone mode.
"We used to think some things were so important that they must be kept constant," Carl Johnson of Vanderbilt University said.
"But those metabolic set points are changing as a function of the time of day," he said.
Johnson's team took careful 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 wrote.
The findings are published in the journal Current Biology, a Cell Press publication.