The pattern of desynchronized sleep (DS) occurrence in the rat was studied during exposure to an ambient temperature (Ta) of 0 degrees C for 48 h and during a 12 h recovery period at laboratory Ta (23 degrees C) following the first and second 24 h of cold exposure. The exposure to low Ta induces a DS deprivation which is followed, during recovery, by a clear DS rebound. Both the decrease and the following increase in the amount of DS are due to changes in the frequency rather than in the duration of DS episodes. The frequency distribution of the intervals between the end of one DS episode and the beginning of the next (DS interval) has shown that two populations of DS intervals exist, i.e. short DS intervals (</=3 min) and long DS intervals (>3 min). On the basis of this, two types of DS episodes have been identified: the 'single DS episode', which is both preceded and followed by a long DS interval, and the 'sequential DS episode', which is a DS episode occurring within a cluster or a sequence of DS episodes and is characteristically separated by short DS intervals. The occurrence of such sequential DS episodes in a 'DS cluster', allows a high amount of DS to occur without increasing the duration of the DS episode. DS clusters are repressed during cold exposure, when the DS drive is counteracted by the need to thermoregulate, and enhanced during recovery, when the DS drive is unrestrained. In contrast, the occurrence of single DS episodes is much less affected by such different experimental conditions.