Some studies have even shown that we have two distinct 4 hour natural sleep periods, which in the past would last between 8 PM-12 AM, and then again from 2 AM-6 AM.

一些研究甚至表明，我们有两个4小时的自然睡眠时间，在过去，一段时间会发生在上午8点至12点，另一段时间是凌晨2点至6点。