From the linked wiki article describing sleep in non-humans:

http://en.wikipedia.org/wiki/Sleep_%28non-human%29#Sleep%20d...

Comparative average sleep periods for various mammals (in captivity) over 24 hours

    Horses – 2.9 hours
    Elephants – 3+ hours
    Cows – 4.0 hours
    Giraffes – 4.5 hours
    Humans – 8.0 hours

Glad the avg sleep period of humans in captivity has been measured...