The study in the linked article (and lots of others like it) begs to differ. If sleep was merely convenient rather the biologically necessary, our cognitive functions would not become severely hampered after only a few hours less sleep. Nor would people die after (medically) losing the ability to sleep after x number of days.
Further, if it were true, overweight people very likely wouldn't need to sleep at all.
Further, if it were true, overweight people very likely wouldn't need to sleep at all.