The excitability of the corticospinal system was studied in 23 healthy subjects in sleep stages NREM2, NREM4, REM, and wakefulness using transcranial magnetic stimulation. Assessment of motor thresholds, stimulus-response curves, and latencies of motor evoked potentials shows activation of the fast-conducting corticospinal fibers in all sleep stages and a neuronal recruitment pattern similar to wakefulness, however, at a lower level of excitability and with significant differences between sleep stages.