How many of you have sleep problems? Maybe it’s waking up in the middle of the night and then not being able to get back to sleep, or waking up too early, or nodding off all too often in front of the TV… or, more embarrassingly, during work meetings? One thing’s for sure: our modern world means that we are not sleeping in the way we used to. Derk-Jan Dijk, Distinguished Professor of Sleep and Physiology at the University of Surrey and Director of the Surrey Sleep Centre, says: we are the only species to extend our day using artificial light, and that has consequences. Jim Al-Khalili talks to Derk-Jan Dijk about the many aspects of sleep that he has studied over the last thirty years – such as how circadian rhythms interact with our sleep cycles, and how sleep changes in people with dementia.