Throughout the night, the brain moves between those two phases in 90-minute cycles, with deep sleep becoming shorter and REM periods longer.
The last REM period of the night comes six to seven hours later and it's "very long, very vivid, lots of color, lots of drama and lots of emotion," Cartwright said. "You go from a short preview of the coming attraction to a biggie and it goes from what is currently on your mind to long-term memories."
In 1999, Scott Falater of Arizona was found guilty of stabbing his wife 44 times. Though he never denied killing his wife, Yarmilla, Falater said in his defense that he had a history of sleepwalking.
Falater's neighbor testified that he watched over his backyard fence as the father of two went inside his house to wash his hands, ordered his dog to lie down, then rolled his wife's body into the pool and held her head under water.
He had been trying to fix a faulty swimming pool pump, and defense lawyers suggested his wife may have interrupted him while he was trying again to fix the pump in his sleep, triggering a violent reaction.
In 2007, Nick Walker, 26-year-old British Air Force mechanic whose military nickname was "night walker," for his sleepwalking habits, was found not guilty of raping a 15-year-old during one horrific sleepwalking bout.
Most violent sleepwalkers are men, but Cartwright treated one woman who killed her cat in her sleep. "She was crazy about that cat," she said of her patient.
Standard treatments include the medication clonazepam, which relaxes the muscles during sleep. Other doctors say hypnosis or stress-relieving interventions can work in some cases.
One of the misconceptions about sleepwalking is that somnambulists stumble and fall, but according to Cartwright, they have all their motor skills.
"They are very good at navigating space," she said. "They can go up and down stairs and drive a car. They can navigate in the world, but the face recognition is off."
And that is precisely why these sleepwalkers can murder their cats and wives, because their brains don't know their victims.
And in addition to being "nice people" during their waking hours, sleepwalkers like Falater, have one other trait in common.
"They are overly meticulous, maybe a little bit OCD [obsessive-compulsive disorder]," Cartwright said. "They are doing something good, but then when someone stops them, they turn violent."
If you or a loved one experience sleepwalking, doctors suggest finding a sleep clinic to determine the extent of the parasomnia. Specialists can recommend stress-reduction techniques and medications that can help the disorder.
You can find help at these Web sites: Sleep Health Centers and at the Rush University Medical Center Sleep Disorders Service and Research Center.