Although the relay of sensory information from thalamus to cortex is reduced during sleep, animals can still be woken by noise. The neural circuit that mediates this defence mechanism was unknown, but now Shin et al. have used single-unit recordings from head-fixed mice to show that mediodorsal thalamic (MD) neuronal activity increases during transitions from slow-wave sleep (SWS) to wakefulness. Moreover, photostimulation of MD neurons expressing the ionotropic glutamate receptor subunit GRIK4 induced arousal from SWS, suggesting that the pathway that mediates SWS–wake transitions is glutamatergic.