Electrographic sleep and brain temperature (Tb) were monitored in captive rooks,Corvus frugilegus,for several consecutive 24‐h periods under natural entrained conditions in two different seasons. The courses of brain temperature after the transitions between vigilance states including slow wave sleep (SWS), paradoxical sleep (PS) and wakefulness (W) were calculated. A consistent decline in Tb occurred when birds fall into SWS. During transition from SWS into W a significant rise in Tb was observed. Changes in Tb associated with PS could not be ascertained. The experiments showed the existence of close connections of SWS and W with short term fluctuations superimposed on the daily course of Tb.