Omond Shauni, Ly Linh M T, Beaton Russell, Storm Jonathan J, Hale Matthew W, Lesku John A
School of Life Sciences, La Trobe University, Melbourne 3086, Australia.
School of Psychology and Public Health, La Trobe University, Melbourne 3086, Australia.
Sleep. 2017 Oct 1;40(10). doi: 10.1093/sleep/zsx124.
Sleep either appeared once early in the evolution of animals, or at multiple instances over evolutionary time. Understanding whether sleep is a diagnostic trait for members of the kingdom Animalia has important implications for our understanding of the evolution of sleep and sleep functions. Unfortunately, knowledge on the phylogenetic breadth of sleep is restricted to vertebrates, a few arthropods and molluscs, and one species of nematode. There is a dearth of information on the other 30 or so animal phyla.
Here, we provide original data on a previously unstudied group of animals with respect to sleep: platyhelminth flatworms. These free-living animals are relatively simple, with a rudimentary central nervous system and absence of many other specialized physiological systems.
Despite this simplicity, inactive flatworms appeared to be sleeping. Specifically, quiescence was organized in a circadian manner, occurring largely during the daytime. This basic rhythm persisted under constant darkness, suggesting that it was endogenously generated. Active flatworms responded more readily to stimulation, and flatworms recovered lost sleep by sleeping longer after a 3-hour period of inactivity deprivation. We were also able to increase inactivity in a dose-dependent manner with exposure to melatonin, a hormone that increases sleep in diurnal animals.
Taken together, these data expand our understanding of the phylogenetic extent of sleep and reinforce the idea that sleep evolved early in the evolutionary history of animals. However, additional studies on other types of animals are required for a comprehensive understanding of the origin(s) and evolution of sleep.
睡眠要么在动物进化早期就已出现一次，要么在进化过程中多次出现。了解睡眠是否是动物界成员的一个诊断特征，对于我们理解睡眠的进化和睡眠功能具有重要意义。不幸的是，关于睡眠系统发生广度的知识仅限于脊椎动物、少数节肢动物和软体动物，以及一种线虫。关于其他约30个动物门的信息则十分匮乏。
在此，我们提供了关于一类此前未被研究过的动物睡眠情况的原始数据：扁形动物门的扁虫。这些自由生活的动物相对简单，具有原始的中枢神经系统，且缺乏许多其他专门的生理系统。
尽管结构简单，但不活动的扁虫似乎在睡眠。具体而言，静止状态以昼夜节律的方式组织，主要在白天出现。这种基本节律在持续黑暗的条件下依然存在，这表明它是内源性产生的。活跃的扁虫对刺激反应更迅速，并且在经历3小时的活动剥夺后，扁虫会通过延长睡眠时间来弥补失去的睡眠。我们还能够通过给予褪黑素（一种能增加昼行性动物睡眠的激素），以剂量依赖的方式增加其静止时间。
综上所述，这些数据扩展了我们对睡眠系统发生范围的理解，并强化了睡眠在动物进化历史早期就已出现的观点。然而，要全面理解睡眠的起源和进化，还需要对其他类型的动物进行更多研究。
