The American Time Use survey reports that an average American work day includes 8.8 hours of work and 7.6 hours of sleep. Sleep is the second largest single use of time. However, new drugs such as Modafinil appear to vastly reduce the need for sleep without significant side effects (at least so far). Based on reports from users, it seems that people could realistically cut their sleep requirements to as few as 2.5 hours a night without a decrease in mental acuity. That gives us another 5 hours to distribute over the day.
Source: Sociological Speculation. HT: Tyler Cowen.