S1. You do not remain at this deep fourth stage all night long, but instead about 80 minutes after you fall into slumber, your brain activity level will increase again slightly.

【参考答案】

但你并不是整夜都保持这种第四阶段 的沉睡状态，入睡后大约80分钟左右，你的大脑活动 会再度略有加强。