Global warming has put the energy consumption of all industries into focus. In 2005 mobile communications contributed to about 0.2% of global CO2 emissions. Mobile operators have been reporting annual increases from 300% to 700% in 3G data traffic volumes. Such a steady growth in traffic requires regular upgrades in the infrastructure. While network equipment is in itself becoming more efficient, these upgrades still increase the overall energy consumption of the networks. This paper investigates the energy saving potential of exploiting cell size breathing by putting low loaded cells into sleep mode. The energy consumption and network performance of the resulting network are used to quantify the potential of this feature. The investigation is carried out on a tilt optimized network. Since putting cells into sleep mode results in a non- optimum antenna tilt configuration, this paper also investigates the possible gains of re-optimizing antenna tilting. The results show that by allowing sleep mode, over a restricted period of 12 hours, an energy saving of 33% is possible. While this energy saving comes at no expense of the overall network performance, the gain in average user data rate is noted to decrease by 21%.