Complex sentence: Throughout his sessions he found no sudden onset of irregularity in her sleep, but he observed a few persistent abnormalities, including a significantly reduced amount (and much larger than average variance in duration) of REM sleep and an unusually high number of sleep spindles.
Split into simpler sentences: Throughout his sessions, he found no sudden onset of irregularity in her sleep. However, he observed a few persistent abnormalities, including a significantly reduced amount (and much larger than average variance in duration) of REM sleep and an unusually high number of sleep spindles.