It depends on your genes:
[T]here are a few very rare individuals who can manage with only five hours sleep a night without experiencing deleterious effects. They are sometimes known as the “sleepless elite”. In 2009, a team led by geneticist Ying-Hui Fu at the University of California San Francisco discovered a mother and daughter who went to bed very late, yet were up bright and early every morning. Even when they had the chance to have a lie-in at the weekend (a tell-tale sign that you are sleep-deprived) they didn’t take it.
Tests revealed that both mother and daughter carried a mutation of a gene called hDEC2. When the researchers tweaked the same gene in mice and in flies, they found that they also began to sleep less – and when mice were deprived of sleep they didn’t seem to need as much sleep in order to catch up again.