1.The amount of air exchange is even lower in REM than NREM because, although breathing is more rapid in REM, it is also more irregular, with brief episodes of shallow breathing or absence of breathing.
“The amount of …than NREM”为句子主干，“because…”为原因状语从句，“It is also more…”为句子主要表达的意思，是比较重要的内容。
2.The frequency with which certain simple motifs appear in these oldest sites has led rock-art researchers to adopt a descriptive term—the Panaramitee style—a lable which takes its name from the extensive rock pavements at Panaramitee North in desert South Australia, which are covered with motifs pecked into the surface.
“The frequency”为主语，“which”句子连接词意味着从句，“has led rock-art…”修饰term. “which takes its…”这个句子修饰Label.