Objectives: Epileptic susceptibility is triggered by the sleeping condition. However, both ictal and interictal events are not equally affected by the different sleep states. Besides the well-known dichotomy between non-REM sleep (high activation) and REM sleep (low activation), epileptic phenomena are deeply sensitive to the ongoing level of arousal. Methods: During non-REM sleep the arousal level can be either unstable, as expressed by the repetitive sequences of the cyclic alternating pattern (CAP), or stable, as re¯ected by non-CAP. Phase A (arousal complex) and phase B (post-arousal rebound response) are the two basic components of the CAP cycle, which presents a 20±40 s periodicity. Three subtypes of A phases can be recognized: the A1 subtypes, which are thoroughly composed of K-complexes and delta bursts, and subtypes A2 and A3 dominated by moderate (A2) or prominent (A3) EEG desynchrony. Results: As a manifestation of unstable sleep, CAP offers a favorable background for the occurrence of nocturnal motor seizures that in most cases arise in concomitance with a phase A. In primary generalized epilepsy (PGE) and in lesional epilepsies with fronto-temporal focus, activation of interictal discharges is high during CAP reaching the climax during phase A and the strongest inhibition during phase B. A lack of modulation is observed instead in epilepsy with benign rolandic spikes. In PGE, the interictal bursts are mostly associated with the highly synchronized phase A1 subtypes. Conclusions: The analysis of sleep microstructure based on CAP parameters offers a sensitive framework for exploring the linkage between dynamic EEG events and epileptic phenomena. q 2000 Elsevier Science Ireland Ltd. All rights reserved.