Non-REM sleep electrophysiology in REM sleep behaviour disorder: A narrative mini-review

Isolated Rapid Eye Movement Sleep (REM) behaviour disorder (iRBD) is a prodromal sign of neurodegenerative disorders. Empirical findings point to a role of non-REM (NREM) sleep alterations in neurodegenerative processes. Therefore, the interest in NREM sleep electroencephalography (EEG) of iRBD is progressively increasing. The present review aims to provide an updated state of the art on NREM sleep electrophysiology in iRBD. First, we describe findings on NREM EEG power spectra. Then, we consider specific NREM sleep EEG hallmarks (i.e., slow waves, slow oscillations, K-complexes, sleep spindles). Finally, we focus on NREM sleep instability. The reviewed literature is small and heterogeneous, but rapidly growing. The most consistent findings point to alteration of sleep spindles and cyclic alternating pattern in RBD. A larger discrepancy characterized results on slow wave activity, but recent studies using a topographical approach provide promising results. Evidence on the relationship of NREM sleep alterations with neurodegenerative processes in iRBD, as well as longitudinal changes, are scarce. We discuss the main methodological limitations, highlighting possible future directions.