The amount of sleep varies from individual to individual. The 7-8 hour sleep is just a median number but in reality, it could vary from 3 hours to 14 hours for adults. If you read the biographies of Margaret Thatcher and the ex-chinese premier Deng Xiao Ping, you will be amazed on their sleep habits. I think you need more than 6 hour sleep. I myself need 9 hours.