Cortical direct current (DC) potentials are considered to reflect the state of cortical excitability which may change characteristically from wakefulness to sleep. The present experiments examined changes in the scalp recorded DC potential in 10 healthy humans at the transition from wakefulness to nocturnal sleep. For each subject, DC recordings obtained from Cz were evaluated for a 15 min pre-sleep onset interval and for a 20 min post-sleep onset interval, on 2 separate nights. Sleep stages were determined from standard sleep recordings. The transition from wakefulness to sleep coincided with a significant (P < 0.05) shift in the DC potential of negative polarity. Maximum negative potentials of (mean +/- S.E.M.) 500 +/- 130 microV (first night) and of 760 +/- 200 microV (second night) were reached at the end of the 20 min post-sleep onset interval. A number of possible technical and biological artifacts were controlled. It is reasonable to assume that the slow negative shift of the DC potential at the transition from wakefulness to sleep reflects increased cortical excitability. Whether the negative potential shift pertains during sleep, or is of transient nature and closely linked to the process of falling asleep, remains to be clarified.