We have recorded from extrastriate area V4 in monkeys performing a visual search task. When animals became tired or drowsy, responses to visual stimulation were often reduced or even completely blocked, and background activity changed to the burst-pause pattern typically seen in sleep. In spite of such neuronal sleep observed in V4, animals continued to perform the visual task, indicating that at least the primary visual cortex was still working. This observation shows that sleep does not develop simultaneously in all cortical areas but may affect some areas earlier than others. In particular conditions, local sleep of certain areas may be a stable and long-lasting phenomenon.