Although no one knows for certain the causes of sleepwalking, Dr. Wallace Mendelson, codirector of the National Capital Sleep Center at Bethesda's Suburban Hospital, hypothesizes there are three types of sleepwalkers:

* Children, about age 8-15 (more frequently male). They usually outgrow the condition, which is considered benign.

* People suffering very unusual side effects of drugs such as sleeping pills.

* Those undergoing a seizure of the nervous system, the most serious category. These sleepwalkers are more likely to injure themselves because they are less sensitive to their surroundings.

One of the characteristics of "benign" sleepwalking is very docile behavior, says Mendelson. The subject usually can be led easily back to bed.

With seizure-type sleepwalking, the walker may resist being led back to bed and is more difficult to handle.

Because the condition is most common in children, "The parents, not the patients, are often the ones in need of reassurance," says Mendelson.

Says Dr. William H. Reid of the Nebraska Psychiatric Institute: "There is some theoretical evidence that it may be related to a small part of the nervous system that doesn't mature on time.

"What we do know is that sleepwalking is not associated with serious mental or physical illness. None of the studies we've done indicate a particular psychiatric problem."