The brain is as active during REM (rapid eye movement) sleep—when most vivid dreams occur—as it is when fully awake, says Dr. Eric Nofzinger at the University of Pittsburgh,

看红色部分：as...as... 比较结构。第一个 as 表示程度，指“一样地积极；那样地积极”。第二个as 是比较代词，引出比较从句。