Probing the need for sleep.

To better understand why we humans sleep as we do--and why sleep even exists--scientists are examining the slumber habits of a wide range of animal species.

                        HOURS OF   EYE POSITION
ANIMAL             SLEEP PER DAY   DURING SLEEP

Hairy armadillo             20.4            * * both closed
Brown bat                   19.9            * *
Southern opossum            19.4            * *
Ground squirrel             14.5            * *
Domestic ferret             14.5            * *
Mountain beaver             14.4            * *
Burrowing owl               14.3           * ** one eye sometimes open
Cat                         13.2            * *
Pigeon                      11.9           * **
Domestic chicken            11.8           * **
Chimpanzee                  10.8            * *
Dog                         10.7            * *
Three-toed sloth            10.5            * *
Fruit fly                   10.0          ** ** no eyelids
Human                        7.7            * *
Asiatic elephant             5.3            * *
Cow                          4.0            * *
Horse                        2.9            * *
Giraffe                      1.9            * *
Bullfrog                       0

SOURCES: INDIANA STATE UNIVERSITY, UCLA, & UNIVERSITY OF WASHINGTON