I believe the answer is: they can function well on 6 hours or less of sleep
On average, an adult need around 7-8 hours of sleep in order for their brain to function on optimal Level. Short sleepers on the other hand, could function T
The energy level that short sleepers achieve became the main thing that differentiate short sleepers with sleep deprived (A lot of people in large cities actually receive less than 6 hours slee but it's because they have too much work)