Interictal spiking during wakefulness and sleep and the localization of foci in temporal lobe epilepsy. We examined variations in interictal spiking during sleep and wakefulness to assess differences in reliability for localizing epileptic foci. Forty patients were studied prospectively. Spikes were assessed for rates, field, and appearance of new foci. Final localization was determined by surgery, electrocorticography, and seizure onset. Comparison of interictal EEG foci with final localization was made. In 39 patients, slow-wave sleep activated spiking compared with wakefulness. Most patients showed maximal spiking in sleep stages 3 or 4. Restriction of field in rapid eye movement (REM) sleep and wakefulness, and extension of field in slow-wave sleep occurred. New foci appeared in non-rapid eye movement sleep in 53% of patients. Similar but not identical spiking rates, foci, and field distributions were seen in wakefulness and REM sleep. All REM foci were unilateral. Our findings suggest that localization of the primary epileptogenic area is more reliable in REM sleep than in wakefulness, and in wakefulness more than in slow-wave sleep. 