October 11, 2001

Sleep deprivation will decrease performance and may cause health problems, but the reasons why we sleep are largely a mystery. Terry Sejnowski, a neuroscientist at the Salk Institute in California, talks about how sleep improves our ability to learn, and how our brain uses sleep in memory.

Researchers are studying how other species sleep to learn more about human sleep cycles. Charles Amlaner is a sleep researcher at Indiana State University. He talks about which parts of the brain shut down during rest, and how our body uses sleep. He explains how animals in the wild stay alert while asleep.

Todd talks with historian Amir Aczel about the origins of the compass. Aczel discusses how the compass was invented, and why early researchers studied birds' sense of direction. He explains his search for information about the compass, and how this tool has been used in exploration, trade, and navigation. His book is "The Riddle Of The Compass: The Invention That Changed The World".