Isabelle can literally play the piano in her sleep - she has also been known to sing.

A video of the 13-year-old Napier girl playing a made-up ditty while sleepwalking on Monday night has gone viral online after being posted to YouTube this week.

Her mother's Auckland-based cousin, James Hodgson, viewed the footage on a phone and instantly knew it would be a success on the web due to its unusual nature. He posted it at 1am yesterday and was overwhelmed by the response after checking in later that morning.

"When I saw it, I thought 'that's amazing, I can't believe it - I bet other people would find that interesting too'," Mr Hodgson said.

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"I went to bed, it had 160 hits, and when I woke up it was 13,000 or so."

In less than 24 hours it had more than 20,000 views and was picked up by The Telegraph, ABC and Channel 9 news in Australia, among others. If it reaches a million hits YouTube will pay out $1000 - money Isabelle would use to buy a laptop.

Despite comments claiming the clip must have been "faked", the sleeping sensation insists it is the real deal, though she did not wake up once or remember a thing. "When I saw it, I was just laughing at how I would stop and snore and then keep playing," she said.

The avid singer, who started piano lessons just two terms ago, appeared to be playing a tune though it was not one she recognised. "I think it was a bit of a mix by the sounds of things, it sounded like something I knew, with something I made up - I was there for five minutes straight."

Afterwards, Isabelle was such a dead weight all her mother Renee Welby could do was put a cushion down where she had curled up beside the piano.

"I was in bed when I heard it, I'm thinking 'this is crazy, this is really strange'," Ms Welby said. "I walked in and she was playing, my other daughter got up and said 'we should record this', we thought we could save it for her 21st."

It wasn't totally out of the ordinary for Isabelle, who once dreamed she was on stage with Queen frontman Freddie Mercury singing I Want to Ride my Bicycle - only to realise she had belted out the tune in the dark with her eyes closed. Her sister once found her deep in slumber, standing bolt upright in the hallway. But family know the drill - quietly lead her back to bed without waking her.

Studies show adept musicians develop a high level of muscle memory, allowing them to play their instrument without mentally focusing on physical movements.

Sleep Well Clinic director Dr Alex Bartle said sleep-walking took many forms and generally happened within two to four hours of hitting the hay.

"You can do some quite complex things when you are asleep, to the point of driving a car. If she had been playing a new piece of music the instinct would be there to put fingers on the piano," he said. "We have muscle memory which allows musicians to play without sheet music, it's a subconscious simulation of moving your hands."

Singing was another "basic function" mimicked in sleep but used different functions to the far more common sleep talking.