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

## TL;DR

This study shows how brain activity in monkeys adapts quickly to changes in light-dark cycles while maintaining daily rhythms.

## Contribution

The study reveals rapid adaptation of delta brain waves to environmental shifts in a diurnal species.

## Key findings

- Delta power showed a strong 24-hour rhythm and 57.61% variance explained by time of day.
- Delta power transitioned early after the shift and realigned within 3 days.
- Delta power and activity showed a stable anti-phase relationship across conditions.

## Abstract

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.

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.

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.

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.