Energy management is a critical issue for mobile devices, with network activity often consuming a signiﬁcant portion of the total system energy. In this paper, we propose Catnap, a system that reduces energy consumption of mobile devices by allowing them to sleep during data transfers. Catnap exploits high bandwidth wireless interfaces – which oﬀer signiﬁcantly higher bandwidth compared to available bandwidth across the Internet – by combining small gaps between packets into meaningful sleep intervals, thereby allowing the NIC as well as the device to doze oﬀ. Catnap targets data oriented applications, such as web and ﬁle transfers, which can aﬀord delay of individual packets as long as the overall transfer times do not increase. Our evaluation shows that for small transfers (128kB to 5MB), Catnap allows the NIC to sleep for up to 70% of the total transfer time and for larger transfers, it allows the whole device to sleep for a signiﬁcant fraction of the total transfer time. This results in battery life improvement of up to 2-5x for real devices like Nokia N810 and Thinkpad T60.