Faced by the large number of deployed Wifi Access Points (AP), many research efforts focus on energy savings in Wireless Local Networks. One of the most promising solutions for improving energy efficiency is the Sleep Mode approach, which is especially effective in dense deployments. It is based on switching off the APs while they are not in use, in order to avoid unnecessary energy consumption. In this paper we evaluate the potential of switching off APs using real measurements taken in a dense urban area. We collected traces covering more than 20 hours, confirming the high density of currently deployed APs in such an environment. Based on these traces, we evaluate how many APs can be switched off while maintaining the same coverage. To this end, we propose two algorithms that select the minimum set of APs needed to provide full coverage. We compute several performance parameters, and evaluate the proposed algorithms in terms of the number of selected APs, and the coverage they provide. Our results show that between 4.25% and 10.91% of the detected APs are sufficient to provide the same coverage, depending on the data set, the mobile terminal and the AP selection algorithm.