Well for children anyway:
"The less a child sleeps, the more likely he or she is to become overweight, according to researchers from Université Laval's Faculty of Medicine in an article published in the latest edition of the International Journal of Obesity. The risk of becoming overweight is 3.5 times higher in children who get less sleep than in those who sleep a lot, according to researchers Jean-Philippe Chaput, Marc Brunet, and Angelo Tremblay." [VIA]
There seems to be a direct causal link between short periods of sleep and reduced levels of leptin and higher levels of ghrelin. This seems to cause increased appetite and thus BMI so this association between a lack of sleep and obesity is probably true for everyone.
I suspect a major cause of sleep deprivation in children and adults is excessive television watching and also sadly computer use.
Chaput JP, Brunet M, Tremblay A.Int J Obes (Lond). 2006 Mar 14; [Epub ahead of print]
Relationship between short sleeping hours and childhood overweight/obesity:
results from the 'Quebec en Forme' Project. [LINK]
Taheri S, Lin L, Austin D, Young T, Mignot E.
PLoS Med. 2004 Dec;1(3):e62. Epub 2004 Dec 7.
Short sleep duration is associated with reduced leptin, elevated ghrelin, and
increased body mass index. [LINK - full text]