gangler:

Ah, I’ve finally figured it out. Zoro despite almost always being asleep is so hard to catch off guard because of this. He doesn’t actually sleep as most humans do. He sleeps a very light sleep with minimal REM. This allows him to be constantly alert at all hours of the day awake or asleep, but also requires him to sleep like eighteen hours out of every twenty-four just to break even.

Quite likely he doesn’t even actually have a proper sleep cycle that would extend a full eight hours through the night. Just takes very frequent naps during period of inactivity.

It’s possible that when the crew is out as sea and there’s nothing to be seen across any horizons he sometimes allows himself to sleep in earnest trusting in the crew to handle any surprises thus explaining why he occasionally gets caught off guard asleep or can’t be woken up in those moments but never on dry land.