Reap the Rewards of Intermittent Fasting, and Avoid the Downsides

“Try to eat within 1 to 2 hours of waking,” Sears says. This will prevent you from having a low fasting glucose level for too long, which some studies suggest may raise heart disease risk.

If you have to get up unusually early on occasion, however, it may be better for your blood sugar control to wait until your usual breakfast time to eat, says Andrew McHill, PhD, a research assistant professor at the Oregon Institute of Occupational Health Sciences in Portland. That’s because levels of the sleep hormone melatonin, which rise in the evening and fall in the morning, may still be elevated when you wake up earlier than normal. When melatonin is high, insulin, which is responsible for processing glucose, is reduced.

And breakfast foods, including fruit, eggs, and whole grains such as oatmeal, can boost your intake of fiber or other nutrients, countering insulin resistance and improving glucose tolerance at your next meal. Research also suggests that a large high-protein breakfast—30 grams of protein (e.g., a cup of cottage cheese) and 350 or more calories—may help control appetite and satiety, and support weight control.

Your body is most efficient at processing carbohydrates in the morning and early afternoon, Sears says. That means it’s better to eat foods that may cause glucose spikes earlier in the day.

But try to avoid having sugary foods on an empty stomach. Instead, eat them as part of a meal with protein and fat, which will blunt the treat’s impact on blood sugar. So if you have, say, a banana muffin at breakfast, eat it with a boiled egg.

Research suggests that it’s best to finish eating between 6 p.m. and 8 p.m. As bedtime approaches, melatonin increases and insulin output begins to drop. That means blood sugar climbs higher and circulates for longer because there isn’t enough insulin to clear it quickly. Research has linked late-evening eating to a greater risk of obesity, type 2 diabetes, and heart disease. In a recent study by Johns Hopkins University, for example, healthy people who ate dinner at 10 p.m. saw greater spikes in blood sugar, slower body-fat breakdown, and increases in cortisol, a hormone thought to be involved in weight gain, compared with a group who ate the same meal at 6 p.m.

Most Americans consume almost 45 percent of their total daily calories at dinner and in an after-dinner snack. A healthier goal, Sears says, is 30 percent. That’s 600 calories for someone who usually eats 2,000 calories a day. For example, in 2013, Israeli researchers looked at the effects of eating a small dinner and a large breakfast vs. a large dinner and a small breakfast in a study involving 93 overweight women. Both groups ate the same lunch and the same overall number of calories. After 12 weeks, those who ate the lighter dinners lost more weight, had a smaller waist, and had better metabolic profiles than the women in the other group.

One way to downsize the dinners that you prepare is to make them the healthiest meal of the day. Try eating lots of vegetables, which are naturally low in calories, suggests Courtney M. Peterson, PhD, an assistant professor in the department of nutrition sciences at the University of Alabama at Birmingham. That means you can eat a large portion so that you’re not hungry afterward and still consume fewer than 600 calories. The fiber from those foods will also help you feel fuller.

And if you don’t overdo it on calories in the evening, Peterson says, you can afford to have a bigger breakfast and lunch, when your body is primed for food processing.

For all the reasons mentioned here, “our general rule is ‘no food after dinner,’” says Krista Varady, PhD, a professor of nutrition at the University of Illinois at Chicago. It’s probably the most important change you can make. “I think the lack of nighttime snacking is the main reason we see such great decreases in insulin resistance in time-restricted eating studies,” she says.

If you must eat a snack, have a small portion of a food low on the glycemic index (GI)—a measure of how quickly a food raises blood glucose—such as celery, cucumbers, apples, blueberries, or raspberries. Many typical snacks—cookies, chips, crackers—are high GI, and “research suggests that it is metabolically unhealthy to eat foods with a high glycemic index late in the day,” Peterson says.

Whichever of these strategies you use, employ them consistently. Irregular eating has been linked to a higher risk of metabolic syndrome (health conditions that can lead to heart disease and type 2 diabetes). “The data suggests that maintenance of regular meal times will promote better heart health and metabolism,” Sears says. For example, a study from the Salk Institute for Biological Studies that tracked people’s eating habits found that half of the participants spread their daily meals and snacks over 15 hours or more. When overweight participants in a second part of the study reduced their eating window to 10 to 11 hours a day for 16 weeks, they lost weight, felt more energetic, and slept better.

Adults need at least 7 hours of sleep a night for optimal health, according to the American Academy of Sleep Medicine and the Sleep Research Society. But most fall short, and that can influence eating habits and metabolism.

In one study, 16 people slept for 9 hours a night for five days (to simulate the workweek), then switched to 5 hours a night for five days.

Sleeping just 5 hours disrupted their circadian rhythm, which led them to take in excess calories, gain weight, and experience an almost 20 percent drop in insulin sensitivity.

Insufficient sleep seems to prompt evening eating, McHill says. “And it’s those calories that seem to account for the [resulting] weight gain and metabolic issues.”

What you eat can also disrupt sleep. Consuming spicy foods close to bedtime may upset your digestive system, and caffeine may delay the onset of melatonin. (McHill says to put down your coffee cup at least 5 hours before going to bed.) Research also suggests that eating lots of added sugars and processed grains may trigger insomnia, while eating more vegetables and fruit may help prevent it.

Editor’s Note: A version of this article also appeared in the April 2021 issue of Consumer Reports On Health.