A new study found that infants and toddlers who slept less than 12 hours a day were more than twice as likely to be overweight by age 3, further evidence that how much we sleep affects how much we weigh.
Researchers from Harvard University interviewed moms and weighed 915 babies — beginning at birth and then again at 6 months and 3 years of age.
"Infants and toddlers who slept less than 12 hours a day were twice as likely to become overweight at age 3 than children who slept longer than that," says Elsie Taveras, the pediatrician who led the study.
"Longer" meant about 13 to 14 hours of sleep a day. Other studies have produced similar findings with school-age children and adolescents. They found that the longer they slept, the less likely they were to be overweight. Taveras acknowledges that may seem counterintuitive.
"When we started looking into this relationship, we actually thought the same thing you just mentioned: We thought it would be the opposite — that children who sleep more weren't as physically active, and maybe that would be one way in which they would be overweight," she says.
Instead, Taveras says, it seems that a healthy night's sleep produces the right amount of appetite hormones. That's evident from research done with adults.
In one study, Stanford University psychiatrist Emmanuel Mignot, who specializes in sleep disorders, looked at 1,000 healthy adults who normally slept about eight hours a day. They were brought into a sleep lab and had their sleep interrupted, so they ended up sleeping only five hours a day. Every morning, their blood was taken and hormones measured.
"Somehow, the brain sends signals to the fat cells and to the stomach to change the production of these hormones that regulate appetite to tell them, 'Eat, eat,' " Mignot says. "And that's why probably sleep deprivation stimulates hunger, and that's why people eat more, and that's maybe why when you sleep too little, you have a tendency to gain weight."
Mignot says that's probably the reason why people eat more when they sleep less: Hormones tell them to. That, plus the obvious: If you're sleeping, you're not eating. Speaking in terms of human evolution, Mignot says, that makes a lot of sense.
"In the past, wakefulness — how much you are awake — was devoted to searching for food, and you were burning a lot of calories to get food. You had to run around and had to hit a rhino with — something, I don't know — and then you get the food. So it took some effort, whereas now, it's just opening the fridge."
Who knows how many calories it took to hunt a rhino. We'll probably never know. But Mignot says our human ancestors probably experienced a healthy correlation between activity and food consumption. Today, temptation abounds, and no amount of exercise can compensate for a daily diet of, say, fast food and snacks.
The study on infants, toddlers and weight is published in this week's Archives of Pediatrics and Adolescent Medicine. Other studies in the same issue point to more healthful benefits of sleep, including fewer emotional, mental and physical health problems later in life.