From the article:
In individuals under 40, the study showed a clear association between averaging five hours or less of sleep each night and large increases in visceral fat, or fat around the organs. Of the study participants under 40, Hispanic men and black women were the largest groups to report getting such little sleep.
Oversleeping may have an effect, too, but it was only seen in people under 40:
The researchers found that getting more than eight hours of sleep on average per night has a similar – though less pronounced – affect and is a problem most commonly seen in Hispanic women of all ages. Surprisingly, the connection between extremes of sleep and accumulation of visceral fat was seen only in patients under 40, Hairston said.