Enjoying a relaxing nap can be regarded in many cultures as a sign of laziness, but a recent research seems to support the famous English idiom "Early to bed, early to rise, makes the man healthy, wealthy and wise" ... or at least slimmer.

Because researchers at Northwestern University found that children who sleep more will store less body fat than children who sleep less, and that the former are less likely than the latter to be overweight five years later.

The research team investigated 2,281 children from a nationally representative survey named the Child Development Supplement of the Panel Survey of Income Dynamics. The subjects were aged 3 to 12 at the start of the study, and 8 to 17 when follow-up information was achieved. The research monitored the children's sleep hours, the moments they went to bed and when they woke up.

Children with more sleep hours presented lower BMI (body mass index) and later in their development were less likely to be overweight than those who slept less. The results were similar even when putting in the equation factors like BMI, parents' income and education, race and ethnicity.

The team found that one more hour of sleep translated in a decrease of children's risk of being overweight from 36 % to 30 %, while for older children the drop was from 34 % to 30 %.

The National Sleep Foundation recommends that children aged 5 to 12 should sleep 10 to 11 hours each night, while teenagers 8 to 9 hours, but the research encountered situations when on weeknights, 7-year-olds had less than 10 hours of sleep, while by the age of 14, weekday sleep time dropped to 8.5 hours. "Our results suggest that many American children are not sleeping enough," said lead researcher Emily K. Snell of the School of Education and Social Policy and the Institute for Policy Research at Northwestern University.

"In addition, our results suggest that encouraging parents to put younger children to bed earlier and allowing both younger and older children to sleep longer in the morning, as well as encouraging school districts to avoid very early school start times, might represent one important and relatively low-cost strategy to add to other efforts to help reduce childhood weight problems."