This isn't surprising at all if you see how REM sleep works. In short, you get more REM sleep in every subsequent cycle (each ~90 minutes), but a significant b

This isn't surprising at all if you see how REM sleep works. In short, you get more REM sleep in every subsequent cycle (each ~90 minutes), but a significant break between sleep cycles starts over the process. So getting 9 straight hours of sleep means you get significantly more sleep than getting 6 1.5 blocks.