As well as being interested in where crimes take place (see locality and ecological explanations) sociologists are also interested in when they happen and most crime occurs at night. One reason for this is the "nocturnal economy" - the big growth in pubs and clubs and the selling of alcohol to young people. Many crimes occur when people leave such establishments, especially when everywhere closed at the same time and city centres would suddenly be full of inebriated young men. This theory emerged from a study by Hobbs and Lister (2000). The idea was used as a justification for introducing 24 hours’ licenses in 2005 (although studies suggest the only significant impact on crime of this change was pushing the time of violent crimes back by a few hours).