Lazy people should sleep more. But how can a hard working man be an obese person? A new Japanese research shows that men who sleep less than five hours a night are exposed to turning obese and to having high levels of glucose in the blood, which is the first step towards diabetes.

"Lack of sleep triggers a hormone in the blood which stimulates the appetite (ghrelin, the most powerful appetite stimulant yet discovered). It increases a sense of hunger as well as an appetite for high-calorie food," said lead author, Nihon University medical department associate professor Yoshitaka Kaneita.

The team looked at 21,693 men in 1999 and made a follow up in 2006. Average weight men in 1999 were 1.36 times more likely to turn obese if they slept less than 5 hours a night on average in the subsequent 7 years compared with the category of those who slept more. The likelihood of the short-sleepers to have high blood sugar levels increased by 1.27 times.

There is no similar research made for women. And the recommended daily sleeping time for an adult is 8 hours.

Still, a research made at Northwestern University in 2006 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.

Children with more sleep hours presented lower BMI (body mass index) and later in their development they 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 per 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.