Hypnagogia，是一个对大多数人来说都很陌生的词，即使你是英语专业出身，对于这个词也不一定很了解。同时本文将倾向于一篇学术考究文。
首先让我们认识一下，究竟什么是Hypnagogia？让我们首先看看英文解释：
Hypnagogia is the experience of the transitional state from wakefulness to sleeping humans: the hypnagogic state of consciousness, during the onset of sleep. Mental phenomena that occur during this "threshold consciousness" phase include lucid thought, lucid dreaming, hallucinations, and sleep paralysis.
Hypnagogia指人从清醒到睡眠的过渡状态，即如文题所说的”睡眠临界态“。在这个Hypnagogia状态下可以发生很多广为人知的有趣的心理现象，比如说清醒梦，幻觉和睡眠瘫痪。
Hypnagogia的研究历史十分久远。早期关于Hypnagogia的引用可以追溯到一些名家的作品，比如说：Aristotle, Iamblichus, Cardano, Simon Forman和Swedenborg。著名的Edgar Allan Poe也提到了他的幻象体验：“only when I am on the brink of sleep, with the consciousness that I am so”。（只有当我处于睡眠的边缘状态时才会遇到“幻象”，意识到我是这样的。）
当然这些都是文学上的考究。Hypnagogia真正进入科学领域，成为科学家们研究的对象开始于十九世纪。同时EEG的出现，也为Hypnagogia的研究提供了大量可供参考的数据。
The word hypnagogia entered the popular psychology literature through Dr. Andreas Mavromatis in his 1983 thesis, while hypnagogic and hypnopompic were coined by others in the 1800s and noted by Havelock Ellis.
The term hypnagogic was originally coined by Alfred Maury to name the state of consciousness during the onset of sleep. Hypnopompic was coined by Frederic Myers soon afterwards to denote the onset of wakefulness.
The term hypnagogia is used by Dr. Mavromatis to identify the study of the sleep-transitional consciousness states in general, and he employs hypnogogic (toward sleep) or hypnapompic (from sleep) for the purpose of identifying the specific experiences under study.
我们更感兴趣的是在Hypnagogia出现的一系列生理和心理现象。我们在经历从清醒到睡眠这一过程的时候，会出现各种各样的感官体验。这种体验可以是很模糊的，也可以是及其清晰的，以至于你难以分辨。
首先要说的是视觉效应。
在人们经历Hypnagogia的时候，通常会感受到光幻觉。这里的光幻觉是指在你闭上双眼后，在即将进入睡眠之前会出现视觉现象。光幻觉通常可以表现成随机的斑点，或者是线条，几何，甚至是完整的画面。这种现象在我其它文章中提到可以作为进入梦境的标注。
这种视觉效应所产生的光幻觉可以是单一颜色的，也可以是彩色的，可以是静止的，也可以是运动的，因为我们需要知道的是这种幻觉不是真实的，而是由大脑产生的，我们不能根据我们所认知的真理来描述。
第二种体验是Tetris effect，即俄罗斯效应。
People who have spent a long time at some repetitive activity before sleep, in particular one that is new to them, may find that it dominates their imagery as they grow drowsy, a tendency dubbed the Tetris effect.
这种效应描述的是，但我们在睡眠之前重复地做一件事情，那么我们在经历Hypnagogia时，会再次体会到这种感觉。比如说某人在蹦床上玩耍了很长时间，那么他很有可能在Hypnagogia时会体会到身体上下运动的感觉。这种现象已经超出了视觉的限制，而是体现为身体其它机能的共同感受。
有些人可能会出现睡眠瘫痪。
什么是睡眠瘫痪？我已经在文章《睡眠瘫痪的科学解释》等多篇文章中介绍过这种现象。简单来说，当你处在睡眠瘫痪时，你有意识，可不能移动身体，即“鬼压床”。
睡眠瘫痪现象发生的频率非常高，偶尔可以占到60%。在对中国等一些国家的调查中，20%-60%的人至少经历过一次睡眠瘫痪。
当然，在Hypnagogia状态下还有很多不同的体验，但是我们需要明白的一点是，我们一般倾向于把Hypnagogia划分到睡眠状态。不仅仅因为在Hypnagogia状态下，我们会出现幻觉等现象，更是因为Hypnagogia经常与我们进一步关于梦境的研究挂钩。
引用（可供感兴趣的读者参考的一些论文）：
Myers, Frederic (1903).Human Personality and Its Survival of Bodily Death. London: Longmans.
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