As if being a new mom wasn't hard enough — there's new evidence that mothers who don't get a lot of sleep are more likely to pack on baby weight, according to a new study by Kaiser Permanete and Harvard Medical School/Harvard Pilgrim Health Care.

Researchers found that moms who got less than five hours of sleep a day when their babies were 6 months old were three times more likely to be carrying 11 extra pounds at the child's first birthday than those who get seven hours. The bottom line –- those extra two hours of sleep could make all the difference.

"We've known for some time that sleep deprivation is associated with weight gain and obesity in the general population, but this study shows that getting enough sleep — even just two hours more — may be as important as a healthy diet and exercise for new mothers to return to their pre-pregnancy weight," said lead study author Dr. Erica P. Gunderson, an investigator at the Kaiser Permanente Division of Research, in a news release.

The study also found that mothers who slept fewer hours at one year postpartum than they did at six months postpartum had twice the risk of substantial weight retention. Other studies have shown that persistent sleep deprivation causes hormonal changes that may stimulate appetite.

"With the results of this study, new mothers must be wondering, 'How can I get more sleep for both me and my baby?' Our team is working on new studies to answer this important question," said Dr. Matthew W. Gillman, Department of Ambulatory Care and Prevention at Harvard Medical School/Harvard Pilgrim Health Care, in a news release.

More than 900 women with a median age of 33 participated in the study. Forty-seven percent of the women were first-time mothers, 37 percent had two children and 16 percent had three or more children.

The study was published in the November issue of the American Journal of Epidemiology.