Fifty years ago, scientists discovered that people experience two different types of sleep: REM (rapid eye movement) sleep, during which the eye moves quickly right and left; and a deeper kind of non-REM sleep, during which the eye doesn't move.
五十年前，科學家發現人類睡眠分為兩部分，眼皮下方的眼球出現規則性的左右擺動時為「速眼動睡眠」；睡得很沉、眼球不動時為「非速眼動睡眠」。