A hypnagogic jerk is an involuntary muscle spasm that occurs as a person is drifting off to sleep.

　　“睡眠痉挛”指的是一个人在入睡时，不随意肌发生突然的抽搐颤动。

　　The phenomenon is so named in reference to the hypnogogic state — the transitional period between wakefulness and sleep. Hypnagogic jerks are also commonly known as hypnic jerks or sleep starts.

　　这个现象一般发生在入睡期——即清醒到睡熟之间的这段过渡时间。睡眠痉挛也俗称为入睡抽动和睡眠开始。

　　The muscle spasms may occur spontaneously or may be induced by sound, light or other external stimuli. Some people report hypnagogic jerks accompanied by hallucinations, dreams, the sensation of falling, or bright lights or loud noises coming from inside the head.

　　这种肌肉痉挛可能是自发的，也可能是受到声、光等外部刺激而引起的。有人报告称，睡眠痉挛还往往伴随着幻觉、梦境和跌落的感觉，有时大脑里面还会感觉到有亮光和吵闹的声音。

　　Sleep starts are quite common, with some research suggesting that 60 to 70 percent of people experience them. Many individuals may be visited by nightly hypnic jerks without even knowing it, as the twitches often go unremembered, particularly if they don’t cause a person to wake up.

　　睡眠痉挛很常见，某些研究甚至称60%到70%的人体验过这种感觉。许多人可能甚至都不知道自己睡觉时发生了睡眠痉挛，因为人们一般都不会记得这种身体的抖动——尤其是如果人们没有因为这个现象而醒过来的话。

　　Some scientists believe certain factors, such as stress, anxiety, fatigue, caffeine and sleep deprivation, may increase the frequency or severity of hypnagogic jerks, but conclusive research is lacking on the subject.

　　一些科学家认为某些因素，例如压力、焦虑、疲劳、咖啡因和失眠等，会加大睡眠痉挛的频率和严重程度，但是却缺乏具有结论性的研究。

　　Researchers are also unsure why hypnic jerks occur, but a few theories exist.

　　目前研究人员们还不确定为什么会发生睡眠痉挛，但的确存在一些这方面的理论。

　　One hypothesis says that hypnagogic jerks are a natural part of the body’s transition from alertness to sleep, and occur when nerves "misfire" during the process.

　　一则假说认为睡眠痉挛是人体从清醒过渡到睡眠自然而然的一部分，而在这个过程中当神经“失败”的时候就会出现睡眠痉挛。

　　Another popular idea takes a more evolutionary approach to hypnic jerks, explaining that the spasms are an ancient primate reflex to the relaxation of muscles during the onset of sleep — the brain essentially misinterprets the relaxation as a sign that the sleeping primate is falling out of a tree, and causes the muscles to quickly react.

　　而另一个流行观点则用更加进化的方法来解释睡眠痉挛，称这种抽搐现象是古灵长类动物在睡眠时松弛肌肉——而大脑却误判了这种松弛，认为这是睡熟的灵长类动物从树上掉下来了的信号，因此发送信号导致肌肉作出快速反应。

（编辑：何莹莹）