Birds and mammals share specialized forms of sleep including slow wave sleep (SWS) and rapid eye movement sleep (REM), raising the question of why and how specialized sleep evolved. Extensive prior studies concluded that avian sleep lacked many features characteristic of mammalian sleep, and therefore that specialized sleep must have evolved independently in birds and mammals. This has been challenged by evidence of more complex sleep in multiple songbird species. To extend this analysis beyond songbirds, we examined a species of parrot, the sister taxon to songbirds. We implanted adult budgerigars (Melopsittacus undulatus) with electroencephalogram (EEG) and electrooculogram (EOG) electrodes to evaluate sleep architecture, and video monitored birds during sleep. Sleep was scored with manual and automated techniques, including automated detection of slow waves and eye movements. This can help define a new standard for how to score sleep in birds. Budgerigars exhibited consolidated sleep, a pattern also observed in songbirds, and many mammalian species, including humans. We found that REM constituted 26.5% of total sleep, comparable to humans and an order of magnitude greater than previously reported. Although we observed no spindles, we found a clear state of intermediate sleep (IS) similar to non-REM (NREM) stage 2. Across the night, SWS decreased and REM increased, as observed in mammals and songbirds. Slow wave activity (SWA) fluctuated with a 29-min ultradian rhythm, indicating a tendency to move systematically through sleep states as observed in other species with consolidated sleep. These results are at variance with numerous older sleep studies, including for budgerigars. Here, we demonstrated that lighting conditions used in the prior budgerigar study—and commonly used in older bird studies—dramatically disrupted budgerigar sleep structure, explaining the prior results. Thus, it is likely that more complex sleep has been overlooked in a broad range of bird species. The similarities in sleep architecture observed in mammals, songbirds, and now budgerigars, alongside recent work in reptiles and basal birds, provide support for the hypothesis that a common amniote ancestor possessed the precursors that gave rise to REM and SWS at one or more loci in the parallel evolution of sleep in higher vertebrates. We discuss this hypothesis in terms of the common plan of forebrain organization shared by reptiles, birds, and mammals.

鸟类和哺乳动物都有特殊的睡眠形式，包括慢波睡眠（SWS）和快速眼动睡眠（REM），这就提出了特殊睡眠为何以及如何进化的问题。先前的大量研究得出结论，鸟类睡眠缺乏哺乳动物睡眠的许多特征，因此鸟类和哺乳动物的特殊睡眠必定是独立进化的。多种鸣禽物种的睡眠更为复杂的证据对这一观点提出了挑战。为了将这种分析扩展到鸣禽之外，我们研究了一种鹦鹉，它是鸣禽的姐妹分类单元。我们在成年虎皮鹦鹉（Melopsittacus undulatus）中植入脑电图（EEG）和眼电图（EOG）电极来评估睡眠结构，并在睡眠期间对鸟类进行视频监控。通过手动和自动技术对睡眠进行评分，包括自动检测慢波和眼球运动。这可以帮助定义如何评估鸟类睡眠的新标准。虎皮鹦鹉表现出巩固的睡眠，这种模式在鸣禽和包括人类在内的许多哺乳动物物种中也观察到。我们发现 REM 占总睡眠的 26.5%，与人类相当，并且比之前报道的要高一个数量级。尽管我们没有观察到纺锤体，但我们发现了与非快速眼动 (NREM) 第 2 阶段类似的明显的中间睡眠 (IS) 状态。在整个晚上，SWS 减少，快速眼动增加，正如在哺乳动物和鸣禽中观察到的那样。慢波活动 (SWA) 以 29 分钟的超日节律波动，表明有系统地进入睡眠状态的趋势，正如在其他具有巩固睡眠的物种中观察到的那样。这些结果与许多较早的睡眠研究（包括虎皮鹦鹉的研究）不一致。在这里，我们证明了之前虎皮鹦鹉研究中使用的照明条件（以及老年鸟类研究中常用的照明条件）极大地扰乱了虎皮鹦鹉的睡眠结构，这解释了之前的结果。因此，在许多鸟类中，更复杂的睡眠很可能被忽视。在哺乳动物、鸣禽和现在的虎皮鹦鹉中观察到的睡眠结构的相似性，以及最近在爬行动物和基础鸟类中的研究，为以下假设提供了支持：共同的羊膜动物祖先在大脑中的一个或多个位点拥有产生 REM 和 SWS 的前体。高等脊椎动物睡眠的平行进化。我们根据爬行动物、鸟类和哺乳动物共有的前脑组织的共同计划来讨论这一假设。