When the idea of this issue was first conceived the two obvious questions were, whether or not we really need any more reviews on sleep and if there had been enough advances in the past couple of years to merit a whole new issue on sleep. We thought that the answer to both questions was yes. While there have been a number of excellent reviews of various aspects of sleep physiology in the past few years, there were two groups of readers for whom we thought such an issue would be useful. The first group is established sleep researchers, for whom the opinions of colleagues are (almost) always of interest. There remain many outstanding questions that need to be addressed in the field, and it is not always easy to insert opinions into a classical review. The second group is those who simply want to learn more about sleep. These may be researchers who are thinking about moving into the field, or people from other fields who have an interest in sleep. We thought that it would be invaluable for both groups to have a collection of opinion pieces that not only update the current state of play but also give insight into what might be of interest in the future.