Rapid eye movement sleep displays distinct fractal dynamics between phasic and tonic states.

Rapid eye movement (REM) sleep consists of phasic and tonic microstates with unique neurophysiological properties, yet their fractal characteristics remain underexplored. Using Higuchi's fractal dimension (HFD) analysis of electroencephalographic data from healthy adults, this study investigated complexity differences between REM microstates. The results showed that both phasic and tonic REM exhibited significantly lower global HFD values compared to wakefulness, while displaying similar overall complexity levels between microstates. Importantly, phasic REM demonstrated regionally specific reductions in fractal dimensionality, with pronounced decreases observed in frontocentral areas. These localized reductions exhibited a negative association with theta band power, yet remained statistically unrelated to Lempel-Ziv complexity (LZC) measures, indicating that HFD and LZC capture distinct aspects of neural signal organization. The findings reveal that although phasic and tonic REM maintain comparable global complexity, they differ in their spatiotemporal fractal patterns. The association between increased theta power and reduced fractal dimensionality suggests that phasic REM represents a neurophysiological state favoring rhythmic regularity, potentially optimized for internal information processing. These results position HFD as a valuable complementary approach for characterizing REM microstates, with potential applications in elucidating the pathophysiology of sleep disorders.