We spend one third of our lives asleep, yet doctors and scientists still have no complete understanding as to why. It is one of the last great scientific mysteries. This talk describes new discoveries suggesting that, far from being a time when the brain is dormant, sleep is a highly active process critical for a constellation of different functions. These include the importance of sleep for learning, memory and brain plasticity. Furthermore, a role for sleep in intelligently synthesising new memories together will be examined, the result of which is next-day creative insights. Finally, a new role for sleep in regulating emotional brain networks will be discussed, optimally preparing us for next day social and psychological challenges.