If you're interrupted while you're going through the early ones -- if someone bumps you in bed, or there's a sound, or whatever happens -- you don't just pick up where you left off.
如果你在前面的阶段就被打扰了， 可能有人翻身碰到你， 或者是有什么声音，或其他什么事， 你没办法回到这个阶段继续睡。