Some MBPs had a problem consuming up to 3% of charge per hour of sleep, so that might explain the discrepancy of the White MB's sleep pattern during its REM phase.
My iBook could on for days sleeping on one battery charge, but it had only 768MB of RAM installed.
My Unibody 15-inch MBP did take 3% per hour, and my new June 2009 17-inch MBP seems to have a good battery and only uses about 1% per hour, which should amount to roughly 100 hours or 4 days of uninterrupted sleep.