Constant light disrupts biological rhythms and worsens sleep quality but does not elevate blood pressure in female rats.

Constant light (LL) disrupts biological rhythms, although more data are available on circadian than on ultradian rhythms. LL has been linked to elevated blood pressure (BP), although most evidence comes from tail-cuff plethysmography in males. However, in nocturnal animals, LL should suppress activity, increase sleep, and lower BP. Therefore, the aim of this study is to provide a comprehensive analysis of the impact of LL on (1) cardiovascular parameters and sleep and (2) circadian and ultradian variability in female rats. We used telemetry for continuous monitoring of heart rate (HR), BP, and locomotor and sleep-wake activity in female rats exposed to LL for four weeks. LL progressively reduced basal systolic BP and HR and weakened the strength of circadian rhythms. Moreover, the loss of daily variability enhanced the acute cardiovascular response. Spectral analysis revealed disrupted ultradian rhythms, with HR power shifting from longer (~7-9 h) to shorter (~1-3 h) periods and locomotor activity showing a parallel decline, including a complete loss of 7-9 h rhythms by week 4. HR variability and baroreflex analysis showed parasympathetic dominance under LL. Sleep analysis revealed significant sleep disruption, characterised by altered distribution of sleep-wake states, reduced non-REM sleep during the light phase, increased fragmentation, and a complete loss of circadian organisation. LL reduced BP in female rats despite leaving total sleep duration largely unchanged, while markedly disrupting cardiovascular circadian and ultradian variability and sleep architecture. These findings suggest that LL-induced chronodisruption imposes a maladaptive physiological load also in nocturnal rodents.