Believe it or not, different people require different amount of sleep. The general recommendation is 8 hours as it covers most of the population, the same reason why it's recommended to drink 8 glasses of water. I don't think it's been shown precisely why we even need sleep. We know what happens during sleep (brain activity spike, GH released, muscle repaired, etc) but what we do not know is why the body has to be asleep to do all that.

So, with a grain of salt, my advice would be to sleep however much you personally need.