People who knew Zoro were astonished by his ability to fall asleep.

To his crewmates, it was an annoying quirk. They couldn't fathom how a person could fall asleep within seconds, especially after asking someone to tell them a story; or how he could sleep through so much mayhem. Like, for instance, the mayhem caused by everyone rushing about to ensure that the drastically changing weather didn't cause their ship to sink. Or the noise that was ever-present on their ship, courtesy of—well, everyone.

So because they couldn't understand, Zoro often found himself awakened by a fist or a foot, or, on rare occasion, a stray rubber hand going fast enough and far enough to launch him into the ocean.

Come to think of it, Luffy never really made fun of Zoro for his sleeping habits. But then, Luffy had a narcoleptic older brother who had the ability to fall asleep mid-sentence, while eating, so he was probably used to it.

And, if a person really thought about it, it actually made sense.

See, Zoro spends, oh, 90% of his waking time training. Ok, 75%. 80%? The point is, the guy spends A LOT of his time training.

In addition, his training consists of lifting weights that are bigger than he is. And because he is Zoro, he drives himself to the breaking point, tells himself he's weak, and pushes past it to train twice as long and twice as hard, harder than should be even possible for any normal human being.

So the swordsman actually has good reason to sleep so much and so soundly. He uses up so much energy in his waking hours that he can sit down and just crash for as long as he wants. It's probably also his body's way of compensating for all the abuse it gets on a daily basis. He needs the time to recharge.

He also needs the time to block things out.

See, losing your longtime rival and the closest thing you have to a best friend and/or crush is pretty traumatic for anyone. For a little kid, it's devastating. Because although kids understand some things the way adults never seem to be able, there are some things that they have a hard time wrapping their minds around.

Things like death.

Most kids—well, at least most kids in Zoro's sort of situation, other aren't so fortunate—most kids don't have to deal with death.

Death is for old people. Young people don't get sick easily. Young people don't die easily. Young people are fighters. Young people are more elastic, because they haven't had the wear and tear of life grind them down til they're old and tired and not as adventurous or stupid.

Young people aren't supposed to die. And because they aren't supposed to die, young people aren't prepared for death.

Young people live in the moment, where everyone is still around, and the people they know they've known forever, and the new acquaintances quickly become old, because when you've only lived for several years a few weeks is a much larger chunk of your life than if you've lived for decades.

So young people don't expect anyone to die because in the now, everyone is alive and since a young person lives in the now and doesn't really look toward the future, now is all that matters and the future doesn't exist.

And because right now this person is alive, and because the now is all that matters, this person will not die because the future does not exist.

Too bad life doesn't work that way.

So Zoro learned that sleep was good because when you slept, you could block out the world and you didn't have to think or feel the pain of loss anymore, because you were asleep.

He hated that kind of pain. Bodily pain was something that didn't even matter to him, so long as it didn't hamper his movements too much, but he didn't know how to deal with mental or emotional or spiritual pain.

He fought with his body, not his brain.

And no one was inflicting this sort of pain so he couldn't cut down the cause of it, and he didn't know how to get to the root, so he trained and slept and trained and slept and trained and slept and tried to avoid the pain, because unlike bodily pain, mental and emotional and spiritual pain sometimes never heals.

Sometimes, it just gets worse.

So he trained to forget and slept to block things out, and worked hard to keep his promise.

Only, sometimes sleeping to block things out didn't work.

Sometimes he dreamt of Kuina.

She had been so full of life, and such a great swordsman, and she had been everything he was striving towards, and dammit, who the hell dies by falling and hitting their head?

So he trained ever harder to forget the pain, and because he trained harder, he slept sounder.

And if anyone on that ship had stopped to actually think about it, they would have realized that Zoro's sleeping habits weren't surprising after all.