Scalp recorded direct current (DC) potentials and their topographical distribution (F3, F4, C3, C4 and Pz) were investigated at the transition from wakefulness to sleep, and during NREM-REM sleep and REM-NREM sleep transitions in 11 healthy men during normal sleep. Changes in endexpiratory CO2 partial pressure (ETCO2), scalp temperature and polysomnographic data were simultaneously measured. The transition to sleep was associated with a negative potential shift reaching significance bilaterally at frontal and central sites 5 to 10 minutes after sleep onset (p < 0.05). Largest shifts were found at frontal locations 10 to 15 minutes after sleep onset averaging (mean ± SEM) -512 ± 103 μV. Negative DC potential shifts also occurred at the transitions from NREM to REM sleep and from REM to NREM sleep, but were, however, less pronounced. The negative DC shift at NREM-REM sleep transitions preceded the REM sleep onset (assessed conventionally by polysomnographic criteria) by about 1.5 minutes. Changes in temperature, ETCO2 and eye movements were ruled out as factors significantly contributing to the generation of these shifts. Assuming a neuronal origin of the DC potential, the negative shifts at the wake-sleep transition and between NREM and REM sleep suggest a temporary phase of increased cortical excitability.