Homeostatic sleep regulation in the absence of the circadian sleep-regulating component: effect of short light-dark cycles on sleep-wake stages and slow waves.

ABSTRACT

BACKGROUND: Aside from the homeostatic and circadian components, light has itself an important, direct as well as indirect role in sleep regulation. Light exerts indirect sleep effect by modulating the circadian rhythms. Exposure to short light-dark cycle (LD 11, 11 h light - dark) eliminates the circadian sleep regulatory component but direct sleep effect of light could prevail. The aim of the present study was to examine the interaction between the light and the homeostatic influences regarding sleep regulation in a rat model. METHODS: Spontaneous sleep-wake and homeostatic sleep regulation by sleep deprivation (SD) and analysis of slow waves (SW) were examined in Wistar rats exposed to LD11 condition using LD1212 regime as control. RESULTS: Slow wave sleep (SWS) and REM sleep were both enhanced, while wakefulness (W) was attenuated in LD11. SWS recovery after 6-h total SD was more intense in LD11 compared to LD1212 and SWS compensation was augmented in the bright hours. Delta power increment during recovery was caused by the increase of SW number in both cases. More SW was seen during baseline in the second half of the day in LD11 and after SD compared to the LD1212. Increase of SW number was greater in the bright hours compared to the dark ones after SD in LD11. Lights ON evoked immediate increase in W and decrease in both SWS and REM sleep during baseline LD11 condition, while these changes ceased after SD. Moreover, the initial decrease seen in SWS after lights ON, turned to an increase in the next 6-min bin and this increase was stronger after SD. These alterations were caused by the change of the epoch number in W, but not in case of SWS or REM sleep. Lights OFF did not alter sleep-wake times immediately, except W, which was increased by lights OFF after SD. CONCLUSIONS: Present results show the complex interaction between light and homeostatic sleep regulation in the absence of the circadian component and indicate the decoupling of SW from the homeostatic sleep drive in LD11 lighting condition.