There is 'compelling evidence' that kids who are deprived of the recommended hours of sleep - ranging from 14 hours a day for a one-year-old to 11 hours for a five-year-old - are more likely to be obese at the age of seven.
Study lead author Dr Elsie Taveras, chief of General Pediatrics at Mass General Hospital for Children in the U.S., said: "Our study found convincing evidence that getting less than recommended amounts of sleep across early childhood is an independent and strong risk factor for obesity.
"Contrary to some published studies, we did not find a particular 'critical period' for the influence of sleep duration on weight gain.
"Instead, insufficient sleep at any time in early childhood had adverse effects."
The study found that, overall, children with the lowest sleep scores had the highest levels of all body measurements reflecting obesity and adiposity, including abdominal fat which is considered to be particularly hazardous.
Dr Taveras added: "While we need more trials to determine if improving sleep leads to reduced obesity, right now we can recommend that clinicians teach young patients and their parents ways to get a better night's sleep - including setting a consistent bedtime, limiting caffeinated beverages late in the day and cutting out high-tech distractions in the bedroom.
"All of these help promote good sleep habits, which also may boost alertness for school or work, improve mood and enhance the overall quality of life."