Fur Seals Suppress REM Sleep for Very Long Periods without Subsequent Rebound.

Virtually all land mammals and birds have two sleep states: slow-wave sleep (SWS) and rapid eye movement (REM) sleep. After deprivation of REM sleep by repeated awakenings, mammals increase REM sleep time, supporting the idea that REM sleep is homeostatically regulated. Some evidence suggests that periods of REM sleep deprivation for a week or more cause physiological dysfunction and eventual death. However, separating the effects of REM sleep loss from the stress of repeated awakening is difficult. The northern fur seal (Callorhinus ursinus) is a semiaquatic mammal. It can sleep on land and in seawater. The fur seal is unique in showing both the bilateral SWS seen in most mammals and the asymmetric sleep previously reported in cetaceans. Here we show that when the fur seal stays in seawater, where it spends most of its life, it goes without or greatly reduces REM sleep for days or weeks. After this nearly complete elimination of REM, it displays minimal or no REM rebound upon returning to baseline conditions. Our data are consistent with the hypothesis that REM sleep may serve to reverse the reduced brain temperature and metabolism effects of bilateral nonREM sleep, a state that is greatly reduced when the fur seal is in the seawater, rather than REM sleep being directly homeostatically regulated. This can explain the absence of REM sleep in the dolphin and other cetaceans and its increasing proportion as the end of the sleep period approaches in humans and other mammals. Lyamin et al. show that although fur seals have 80 min of REM sleep a day on land, they have little or no REM sleep when in water for as long as 2 weeks. When they return to land they show little or no REM rebound. Their switch to from bilateral to unihemispheric nonREM sleep in water may make REM sleep unnecessary.

几乎所有陆地哺乳动物和鸟类都有两种睡眠状态：慢波睡眠（SWS）和快速眼动（REM）睡眠。通过反复唤醒剥夺快速眼动睡眠后，哺乳动物会增加快速眼动睡眠时间，这支持了快速眼动睡眠是受内稳态调节的观点。一些证据表明，连续一周或更长时间剥夺快速眼动睡眠会导致生理功能失调并最终死亡。然而，将快速眼动睡眠缺失的影响与反复唤醒的压力区分开来是困难的。北海狗（Callorhinus ursinus）是一种半水生哺乳动物。它可以在陆地和海水中睡觉。海狗的独特之处在于它既表现出大多数哺乳动物所具有的双侧慢波睡眠，也表现出先前在鲸目动物中报道的不对称睡眠。在此我们表明，当海狗待在海水中（它一生的大部分时间都在此度过）时，它会数天或数周没有快速眼动睡眠，或者快速眼动睡眠时间大幅减少。在这种几乎完全没有快速眼动睡眠之后，当它回到基线条件时，其快速眼动睡眠反弹极小或没有反弹。我们的数据与这样一种假说相符，即快速眼动睡眠可能是为了逆转双侧非快速眼动睡眠所导致的脑温降低和新陈代谢减缓的影响，当海狗在海水中时，双侧非快速眼动睡眠这种状态会大幅减少，而不是快速眼动睡眠直接受内稳态调节。这可以解释海豚和其他鲸目动物没有快速眼动睡眠，以及在人类和其他哺乳动物中随着睡眠期结束临近快速眼动睡眠所占比例增加的现象。 利亚明等人表明，尽管海狗在陆地上每天有80分钟的快速眼动睡眠，但它们在水中长达2周的时间里几乎没有或完全没有快速眼动睡眠。当它们回到陆地时，它们几乎没有或完全没有快速眼动睡眠反弹。它们在水中从双侧非快速眼动睡眠转换为单侧非快速眼动睡眠可能使得快速眼动睡眠变得不必要。