Many diets focus on what to eat, but intermittent fasting is all about when you eat.

With intermittent fasting, you only eat during a specific time. Research shows fasting for a certain number of hours each day or eating just one meal a couple days a week may have health benefits.

Johns Hopkins neuroscientist Mark Mattson has studied intermittent fasting for 25 years. He says our bodies have evolved to be able to go without food for many hours, or even several days or longer. In prehistoric times, before humans learned to farm, they were hunters and gatherers who evolved to survive — and thrive — for long periods without eating. They had to: It took a lot of time and energy to hunt game and gather nuts and berries.

With internet, TV and other entertainment available 24/7, many adults and children stay awake for longer hours to watch TV, scroll through social media, play games and chat online. That can mean sitting and snacking all day — and most of the night.

Extra calories and less activity can mean a higher risk of obesity, type 2 diabetes, heart disease and other illnesses. Scientific studies are showing that intermittent fasting may help reverse these trends.