Mercury
Sleepwalking (somnambulism) is a fairly common 'arousal' disorder, most often seen in children. About 10% of adults experience the condition, which is not harmful but does pose some physical risks and as you've experienced, some loss of sleep.
Sleepwalking episodes can last from 5 minutes to an hour and can include many physical activities including walking outside. Violence can occur if the sleepwalker is obstructed or disoriented by sudden awaking. In England, a man killed another man while sleepwalking. Rare, obviously.
Hormones play a part in sleepwalking as does stress but doctors don't really know what causes it, although it does run in families.
I'm no expert. I got most of this information from the Sleep Channel website's section on sleepwalking.
http://www.sleepdisorderchannel.net/...ng/causes.shtm
Check it out. You're not alone.