Reader Gabrielle wrote in to ask: “Why can't we wake a sleepwalker? Is it a can't do, or a shouldn't do?”
Despite urban legends claiming that waking a sleepwalker will send them into shock or give them a heart attack, it’s pretty much harmless. While you can wake them up, you probably shouldn’t, and that’s for both your benefit.
Waking a sleepwalker has never been shown to cause them any harm, but there’s plenty of potential for the person doing the waking to get hurt. Sleepwalking usually occurs during Stage 3 non-rapid eye movement sleep, also known as slow wave sleep, and this stage of sleep is very deep. While waking straight from stage 3 is difficult, it can be done—but doing so can leave a person in a state of cognitive impairment (sleep scientists call it sleep inertia) for around 30 minutes.
For the rest of the story: http://mentalfloss.com/article/49053/why-arent-you-supposed-wake-sleepwalker