短语讲解：

1. They also do not know why the extra brain activity, at least during the first phase of deep sleep, is always on the left hemisphere.

at least 至少
例句：For that day at least he lost heart.
至少这一天他是垂头丧气的。

2. But it may not help with sleep. That extra brain activity, at least according to this new research, makes it harder to get the sleep people need to wake up well rested in the morning.

wake up 醒来; 活跃起来; 引起注意
例句：Call him if the manager doesn' t wake up in time.
你们经理到时醒不来,就叫醒他。
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