that causes you to lose consciousness for many hours at a time

and periodically experience paralysis and hallucinations?

What I've described are the symptoms of sleep.

If you live to be 80, you will spend around 26 years of your life in this state.

Yet scientists still don't agree on why.

Some say we shouldn't ask why we sleep,

but why we are awake when we don't need to be,

using precious energy and possibly getting into trouble.

The big brown bat is only awake for around 4 hours just after the dusk.

which maximizes its chances of eating and

minimizes its chances of being eaten.

But large herbivores have to spend a lot of time foraging

and have a few places to hide.

So they can't afford to sleep for more than a few hours a day.

If there is some universal function of sleep,

a goat somehow accomplishes it in 5 hours,

a tiger in 15 and a dolphin with one eye open.

Still there's plenty of evidence that

slumber is more than just than laying low.

The whole host of body functions are enhanced during sleep.

From tissue repair,

to immune function,

to blood sugar control.

But the most immediate consequence of

sleep deprivation is that

we can't think as well.

Studies show that people who sleep

before and after learning something new

perform better on tests on memory

and problem solving.

This may indicate that

sleeping brain reinforces and archives new memories.

But there's also evidence that

sleep weakens the connections

between brain cells, in a way that

it keeps important information,

while reducing the clutter.

But how exactly this happens remains unknown.

And it doesn't explain

why some animals sleep much longer than others.

Ultimately, some combination of these ideas

would probably explain why

we spend a third of lives unconscious.

For Scientific American's Instant Egghead, I'm Joss Fong.