Our grandparents may have been onto something with those early-bird dinners. The best time to eat the evening meal is four hours before bedtime. According to a statistician who analyzed time-use data from the U.S. Bureau of Labor Statistics, in America, dinnertime is 6:19 p.m., but it varies from a little after 5 p.m. to after 8 p.m., depending on the part of the country.

Nutritionists vary on when dinner should be served but agree that it should be at least two hours before bedtime. The most critical factor in the dinnertime calculus is melatonin, the hormone that signals it’s time to sleep, says Satchidananda Panda, a professor at the Salk Institute for Biological Studies.

Melatonin, which starts to rise about three hours before we go to bed, also tells the pancreas to cut back on insulin production. If we have a sugar spike after eating late, our body has a more challenging time regulating blood sugar, Panda says. This could put us at risk of diabetes and other metabolic disorders. For that reason, he says the ideal dinnertime is three to four hours before bed. What we eat for dinner matters, too. Slower-digesting food like meat keeps us full longer.

“Only in the last hundred years have we seen easily digestible, highly processed food,” Panda says, explaining that nearly 70 percent of the calories we eat now come from carbohydrates. The modern American diet makes timing your family dinners even more critical. Late-night eating can cause the body to store more fat and reduce levels of leptin, the hormone that tells your brain when you’re full, according to a 2022 study from researchers at Brigham and Women’s Hospital in Boston and the University of Chicago.

A 2018 study from endocrinologists in Japan showed that people with Type 2 diabetes who ate dinner after 8 p.m. had poorer blood-sugar control. Eating too late can affect sleep quality, which can lead to hormonal fluctuations that can cause weight gain, says Amy Kimberlain, a spokeswoman for the Academy of Nutrition and Dietetics. Going to bed on a full stomach can also exacerbate acid reflux, she says.

There are historical reasons for the dinnertime formula, too. The ritual of gathering for dinner began during the hunter-gatherer days when everyone had to be home before dark to get the fire going, Panda says. Hunter-gatherers went to sleep around 9:30 or 10 p.m., as the fire was dying, and woke up at dawn. They needed sustenance to keep them going until they could gather some nuts and berries in the morning. Dinnertime around sundown continued as people began farming.

The diet of hunter-gatherers and early agrarian cultures differed from ours. Dinner was high in protein and fiber with a bit of fat, Panda says. It was hard to digest and, therefore, perfect for a lengthy fast. We also don’t move as much as our forebears. Hunter-gatherers and farmers walked 16,000 to 17,000 steps a day, Panda says, with a significant spike in activity around dusk as they hurried home.

We’re no longer out hunting prey all day—even if work can feel that way at times. The average American now walks just 3,000 to 4,000 steps a day, according to the Mayo Clinic.