Slow wave electroencephalogram spectral properties during adaptation of a new light dark cycle in cynomolgus monkeys.

Study Objectives: Shifts in the light-dark cycle (L:D cycle) often trigger phase shifts in physiological data related to the sleep-wake cycle. Slow wave activity (delta) indicates sleep pressure and intensity. This study examines how delta power adapts to shifts in L:D cycle and the temporal dynamics of its coupling with rest-activity rhythms during re-entrainment. Methods: We collected electroencephalogram (EEG) and accelerometer data from three non-human primates during baseline and shifted (8-h delayed light-on) conditions. We derived delta power (0.5 ~ 4 Hz) using Fast Fourier Transform. To quantify changes in delta power dynamics following L:D cycle shifts, we calculated diurnal differences in delta power, per cent variance explained by time-of-day, circadian coupling with physical activity, and delta power activity transitions timing. Results: In both conditions, delta power exhibited a robust 24-h periodicity, and a significant portion of the variance (57.61% ± 6.99%) could be explained by time of day. We found an early transition of delta power in the first 2 days of the shifted condition, followed by realignment to the light-off time within 3 days after the shift. We used coherence analysis to reveal strong coupling between delta power and locomotor activity, with a consistent anti-phase relationship across baseline and phase shifted conditions. Conclusions: Our findings demonstrate that delta power adapts rapidly to environmental phase shifts while maintaining circadian rhythmicity and stable coordination with rest-activity rhythms. Here, we provide new insight into how neural and behavioral states remain aligned during circadian disruptions in a diurnal species.