As obesity rates soar worldwide, the antidote may seem obvious: Eat less! Move more! But the common-sense approach hasn’t been terribly effective, prompting some scientists to question the simplicity of the formula.

Although personal decisions and self-control certainly play a large role in weight gain, a burgeoning area of research suggests that other factors, from air pollution to sleep deprivation, might be subtly helping humans pack on the pounds.

Obesity, it turns out, is a complex condition that involves multiple genes and pathways. Scientists are finding that certain exposures or conditions can change the body’s metabolism, disrupt the trillions of microbes working in the gut and alter hormonal levels in ways that affect a person’s weight.

Sleeping less than six hours a night doesn’t just make people irritable. It can mess up the systems in the brain that regulate metabolism, said the University of Chicago’s Kristen Knutson, an assistant professor of medicine who recently reviewed published research on the topic in the American Journal of Human Biology.

Sleep deprivation can alter the circulating levels of the hormones that control hunger. “For example, after a short sleep, ghrelin, which stimulates appetite, is higher, and leptin, which signals satiety or fullness, is lower,” Knutson said.

Sleep-restricted people also report greater levels of hunger, particularly for calorie-dense foods, Knutson said.

A lack of sleep may also decrease resting metabolic rates, or the number of calories burned while doing nothing, said sleep loss expert Dr. Orfeu Buxton, a neuroscientist at Brigham and Women’s Hospital in Boston.