Acute effects of exogenous melatonin have been widely reported to promote sleep or induce drowsiness in human. However, testing of the hypnotic effects of melatonin in nocturnal rodents has yielded contradictory results. The latter may be associated with differences in concentration, lighting conditions, time of administration of melatonin, and possibly the type of analysis. In this study, electroencephalogram (EEG) and electromyogram were recorded in pigmented male Brown Norway rats under both light-dark (LD) and constant dark (DD) conditions. Melatonin was administered intraperitoneally at a moderate dose of 3 mg/kg, at either 1 h after lights on under LD condition or 1 h after the activity offset under DD condition. The dosage is known to be able to entrain nocturnal rodents in DD conditions, but does not change sleep in rodents in LD. Only the rats under DD conditions showed a significant reduction in nonrapid eye movement (NREM) sleep latency, while the NREM sleep power spectrum remained unaffected. Under LD condition, melatonin did not alter NREM and rapid eye movement (REM) sleep latency, and had only minor effects on the NREM sleep EEG. Regardless of lighting conditions, melatonin administration resulted in less, but longer episodes for all vigilance states suggesting increased vigilance state consolidation. In the discussion, we compare our results with a summary of previously published data concerning the hypnotic effects of melatonin in polysomnographic/EEG-confirmed sleep in humans and nocturnal rodents. In conclusion, the hypnotic effect of exogenous melatonin in nocturnal rodents not only depends on the time of day, and concentration, but is also influenced by the lighting conditions. Regardless of inducing sleep or not, melatonin may consolidate sleep and through that enhance sleep quality.

中文翻译：

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褪黑激素的催眠作用取决于大鼠的环境照明条件

外源性褪黑激素的急性作用已被广泛报道，可促进人类睡眠或诱导困倦。然而，褪黑激素对夜间活动的啮齿动物的催眠作用的测试却得出了相互矛盾的结果。后者可能与浓度、光照条件、褪黑激素给药时间以及可能的分析类型的差异有关。在这项研究中，在明暗（LD）和恒定黑暗（DD）条件下记录了有色雄性棕色挪威大鼠的脑电图（EEG）和肌电图。在LD条件下开灯后1小时或在DD条件下活动抵消后1小时，以3mg/kg的中等剂量腹腔内施用褪黑素。已知该剂量能够在 DD 条件下吸引夜间活动的啮齿动物，但不会改变 LD 条件下啮齿动物的睡眠。只有 DD 条件下的大鼠表现出非快速眼动 (NREM) 睡眠潜伏期显着减少，而 NREM 睡眠功率谱不受影响。在LD条件下，褪黑激素不会改变NREM和快速眼动（REM）睡眠潜伏期，并且对NREM睡眠脑电图只有轻微影响。无论光照条件如何，褪黑激素的施用都会导致所有警戒状态的发作次数减少但时间更长，这表明警戒状态巩固有所增加。在讨论中，我们将我们的结果与先前发表的关于褪黑激素在多导睡眠图/脑电图证实的人类和夜间啮齿动物睡眠中的催眠作用的数据摘要进行了比较。综上所述，外源性褪黑素对夜行性啮齿动物的催眠作用不仅取决于一天中的时间和浓度，还受到光照条件的影响。无论是否诱导睡眠，褪黑激素都可以巩固睡眠，从而提高睡眠质量。