Development of circadian sleep regulation in the rat: A longitudinal study under constant conditions

Abstract

Study Objectives: To better understand the development of sleep, we characterized the development of circadian rhythms in sleep and wakefulness in the artificially-reared, isolated rat pup using an experimental design that minimized the effects of maternal separation. Methods: Neonatal rats were reared in constant conditions (dim red light) while electroencephalographic and electromyographic signals were continuously recorded for up to 3 weeks. This time period spanned the preweaned and weaned ages. The distribution of sleep-wake states was analyzed to estimate the emergence of circadian rhythms. Results: Overt ∼24-hour rhythms in time spent awake and asleep appear by postnatal day (P)17. A marked bi-modal sleep-wake pattern was also observed, evidenced by the appearance of a pronounced ∼12-hour component in the periodogram over the subsequent 3 days (P17-P21). This suggested the presence of two ∼24-hour components consistent with the dual-oscillator concept. During this 3-day time window, waking bouts became longer resulting in a repartition of the duration of intervals without non-rapid-eye movement (NREM) sleep into short (<30 minutes) and longer inter-NREM sleep episodes. These longer waking bouts did not immediately result in an increase in NREM sleep delta (0.5-4.0 Hz) power, which is an index of sleep homeostasis in adult mammals. The sleep homeostatic response did not fully mature until P25. Conclusions: These results demonstrate that the maturation of circadian organization of sleep-wake behavior precedes the expression of mature sleep homeostasis.