It doesn't take much prior sleep to disrupt memory.这句话为后面的理解而提供.这句位于下面一段之上.

He found that prior sleep of 30 minutes, one hour, two hours, and four hours significantly impaired. If the students were awakedned two or four hours before learning,however,their memory was no longer affected by the sleep.

前面不是说significantly impaired吗?为什么后面又说no longer affected ?

He found that prior sleep of 30 minutes, one hour, two hours, and four hours significantly impaired. If the students were awakedned two or four hours before learning,however,their memory was no longer affected by the sleep.
他发现之前的30分钟、一小时、两小时和四小时的睡眠对健康非常有害。但是，如果学生是在学习前的两小时或四小时被唤醒，那么他们的记忆能力不会受到睡眠的影响。

Sleeping for six hours produced less forgetting than four hours or less of prior sleep.
相比于四小时或者更少的睡眠时间，六小时睡眠造成的遗忘程度要轻一些。