Interference alignment (IA) is a promising technique for interference management in cognitive radio (CR) networks. However, the sum rate may fall short of the theoretical maximum especially at low signal-to-noise ratio (SNR). Moreover, in most of the previous works, energy efficiency (EE) aspect is largely ignored. In this paper, energy-efficient power allocation (PA) and transmission-mode adaptation algorithms are proposed for IA based CR networks. We study a PA scheme aiming at optimizing the EE of networks and sum rate of secondary users (SUs), respectively, guaranteeing the rate of primary user (PU). When SNR is low, PU's rate constraint may be not ensured. Thus SUs may be switched into sleep mode and PU's transmission mode is adapted to achieve its rate constraint when SNR is low. Simulation results are presented to show the effectiveness and efficiency of the proposed algorithms for IA based CR networks.