They burrow for protection. It's harder to get eaten in the ocean while sleeping if you're buried beneath the sand. Other things to know about them is they are jumpers as well. You need a very tight cover on the tank or invariably they will end up on the floor. They are not good fish for beginners or even a lot of experienced reefers. They rarely survive a tank that isn't very well-established with pods, etc. They are often hard to get to eat most foods. Experienced reefers often make sure they have some live food available and spend time weaning them onto frozen or other foods -- and many never will. Finally they often have internal parasites when they first come in. They may eat fine for a few days and look happy and then suddenly die. Only way around this is QT and treatment and they need a QT that caters to their sleeping needs. It's one of those fish that are cheap and LFS's sell the heck out of them -- that they probably shouldn't. If your store took your money and didn't even ask about a sandbed I'd reconsider buying fish there.
