"But the persistence of REM begs the question: Why is it so insistent? When rats are robbed of REM for four weeks they die (although the cause of death remains unknown). Amazingly, even though we spend about 27 years dreaming over the course of an average life, scientists still can't agree on why it's important. Psychiatrist Jerry Siegel, head of the Center for Sleep Research at the University of California, Los Angeles, recently proved that REM is nonexistent in some big-brained mammals, such as dolphins and whales. "Dying from lack of REM is totally bogus," Siegel says. "It's never been shown in any species other than a rat." Some theories suggest that REM helps regulate body temperature and neurotransmitter levels. And there is also evidence that dreaming helps us assimilate memories. Fetuses and babies spend 75 percent of their sleeping time in REM. Then again, platypuses experience more REM than any other animal and researchers wonder why, because, as Minnesota's Mahowald puts it, "Platypuses are stupid. What do they have to consolidate?" But, given that rats run through dream mazes that precisely match their lab mazes, others feel that there must be some purpose or meaningful information in dreams."