Amount Needed

The amount of sleep needed generally depends on the size of an organism. Organisms with smaller body sizes have higher metabolisms, which generates more free radicals (chemicals that can destroy cells). This means that they need more time than others to rebuild and repair their body cells.

For Humans:

Infants generally require about 16 hours a day
Teenagers need about 9 hours on average
Adults need 7 to 8 hours a night

For Animals:

Opossums need 18 hours a day, since they are very small
Ferrets require 14.4 hours per night
Cats need 12.5 hours of sleep
Dogs need 10.1 hours of sleep
Elephants only need 3 hours because of their large body size