Characterization of transition sleep episodes in baseline EEG recordings of adult rats.

By scoring 5-s EEG epochs and calculating spectral power of consecutive EEG segments as short as 1-s, transition sleep (TS) episodes were identified in baseline recordings of adult rats. TS episodes were characterized by the abrupt appearance of theta and alpha waves within an ongoing period of slow-wave sleep (SS). They were followed by paradoxical sleep (PS) or, somewhat more frequently, by a period of wakefulness (W) that often led to an additional SS. Statistical values of the main variables of TS-->(W) and TS-->(PS) episodes are presented, together with comparable data concerning previous SS and following W or PS episodes. On the whole, TS episodes were more numerous than PS episodes, and less numerous than SS episodes. Their average duration was considerably shorter. As a consequence of the identification of TS and of brief W or PS epochs intervening within SS, the number of SS episodes was estimated to be considerably higher than previously assessed, and their average duration considerably shorter.