Pythons, bats and giant armadillos are
among the longest sleepers at over 18 hours a day.
Human babies need 16 hours,
and most of us probably feel
we need around eight hours sleep to function well.
Professor Jerry Seigel from the University of California,
Los Angeles, conducted a study of the sleep times
of a broad range of animals
and found that they vary widely.
Some, like migrating birds,
can survive long periods without sleeping at all.
He believes that shows sleep evolved to conserve energy:
"It's animals that are needlessly active that will not survive,
but animals that are most efficient
and use their waking time to do vital functions,
and are otherwise asleep that will survive."
Sleep helps make best use of limited resources.
In humans, when we're awake,
our brain accounts for 20% of the energy
we use when just sitting around.
Sleeping also makes us less likely to get injured
and less likely to be detected by predators.