Identification of trains of sleep sequences in adult rats

In previous studies based on high resolution EEG analyses of the 7 h baseline session of 18 adult male Wistar rats [6,14], we have identified four sleep sequences initiating with slow wave sleep (SS) and terminating with waking (W) or paradoxical sleep (PS). Two of these sequences contained an intervening episode of transition sleep (TS). Several variables of these sequences (SS-->W, SS-->TS-->W, SS-->TS-->PS, and SS-->PS) were selectively correlated with the capacity of rats to learn a two-way active avoidance task the following day, and were differently distributed in fast learning, slow learning and non learning rats [21]. The temporal organization of different sleep components in sequences suggested that a comparable temporal organization might concern the different sleep sequences, albeit on a longer time scale. We have now used waking periods longer than 60 s to separate clusters of baseline sleep sequences (trains) in the same rats. Trains containing the same sleep sequence (homogeneous trains) have been distinguished from trains containing different sleep sequences (mixed trains). In addition, mixed trains including the SS-->TS-->W sequence (+TSW trains) have been separated from mixed trains lacking that sequence (-TSW trains). Mixed trains of the +TSW type were longest and most numerous, while homogeneous trains were shortest and least abundant. Mixed trains of the -TSW type displayed intermediate values. Several variables of sleep sequences and sleep components differed within mixed trains and among mixed and homogeneous trains. The data indicate that baseline sleep sequences aggregate in relatively long strings in a non random fashion. The mechanism of this association is discussed.