Using extracellular single-unit recordings in nonanesthetized, head-restrained mice, we examined spontaneous and evoked discharges of noradrenaline-containing locus coeruleus (NA-LC) neurons across the sleep-waking cycle. The neurons were all characterized by triphasic broad action potentials. They discharged as either slow (<6 Hz) tonic, single spikes or phasic clusters of spikes specific to wakefulness (W), the discharge rate being highest during active waking and significantly lower during quiet waking. They remained totally silent during both slow-wave sleep (SWS) and paradoxical (or rapid eye movement (REM)) sleep. The phasic unit activity was related to abrupt activation of electromyographic activity occurring either spontaneously or elicited by alerting sensory stimuli. At the transition from waking to sleep, they ceased firing before the onset of cortical synchronization (deactivation), the first sign of electroencephalographic sleep, a significant decrease in firing rate preceding the onset of unit activity of sleep-specific neurons in the basal forebrain (BFB)/preoptic (POA) hypothalamus, as described previously [Takahashi K, Lin JS, Sakai K (2009) Neuroscience 161:269-292]. At the transition from SWS to waking, they fired before the onset of both cortical activation and a significant decrease in activity of sleep-specific neurons. These findings support the previous view that the NA-LC system is involved in both tonic and phasic processes of arousal, and further support our previous proposals that initiation of sleep is caused by decreased activity of waking-promoting neurons (disfacilitation) and that NA-LC neurons play an important role in the sleep/waking switch, that is from waking to sleep and from sleep to waking [Takahashi K, Lin JS, Sakai K (2009) Neuroscience 161:269-292].