Designing energy-efficient cognitive radio transceivers requires joint optimization of medium access control and the physical layer implementation. In this paper we show an energy efficiency optimization strategy for IEEE 802.11n compliant transceivers in terms of energy consumed by the receiver per successfully received bit. To this end, we propose and explore several modifications of a conventional physical layer implementation, all of which target energy proportional behavior. The proposed modifications intentionally include operation modes and algorithm choices that are suboptimal with respect to throughput and error-rate performance. Yet, we show how (under ideal conditions) the rate adaptation at the medium access control layer can exploit these modifications to achieve superior energy efficiency that is 44% below that of a rate adaptation targeting only maximum goodput.