From Sleep Foundation.org :
Sleep walking, formally known as somnambulism, is a behavior disorder that originates during deep sleep and results in walking or performing other complex behaviors while still mostly asleep. It is more common in children than adults and is more likely to occur if a person has a family history of the condition, is sleep deprived, or is prone to repeated night time awakenings.
Stage 3 sleep is also known as N3 or deep sleep, and it is harder to wake someone up if they are in this phase. Muscle tone, pulse, and breathing rate decrease in N3 sleep as the body relaxes even further.
The brain activity during this period has an identifiable pattern of what are known as delta waves. For this reason, stage 3 may also be called delta sleep or slow-wave sleep (SWS).
The Global Village.