Non-random patterns of REM occurrences during REM sleep in normal human subjects: an automated second-order study using Markovian modeling.

An automated analysis of the patterns in REM occurrences during REM sleep in 6 healthy young adults was performed, with an emphasis on second-order parameters. It was found that the majority of REMs were grouped in bursts with a tendency to return to the burst mode once outside of it. The occurrence pattern of REMs within bursts was found not to be of a purely random (renewal) nature, in distinction to that of isolated REMs. Second-order REM occurrence patterns, quantified via Markovian modeling, were not stationary when REM period segments of less than 8 min duration were considered, and those patterns remained fairly constant from REM period to REM period within the night. First-order parameters and non-Markovian second-order parameters showed a less stable behaviour throughout the night. It is concluded that there may exist 2 aspects to REM generation, a relatively unstable (i.e., variable) phasic aspect, quantified by first-order parameters, and a more stable tonic aspect, quantified by second- and possibly higher-order parameters.