When she starts sleeping, she doesn't wake up after a few hours. British schoolgirl Stacey Comerford, 15, can sleep for months.
The condition is called Kleine-Levin syndrome, also known as Sleeping Beauty syndrome.
It is so rare that only about 1,000 people are known to suffer from this condition, The Mirror reported.
Stacey started experiencing symptoms about a year ago, but she was only diagnosed in March as the condition is so rare.
Her mother, Ms Bernie Richards, said her daughter's attacks can come on at any time. The longest lasted two months.
The 53-year-old said: "There's never any warning. I've even found her fast asleep on the kitchen floor.
"When she's in an episode, she might get up to go to the toilet or get a drink, but she's not awake. I call it sleep mode.
"When she wakes, she thinks it's the following day. She doesn't have any memory of it."
The mother of six said the change in her daughter is like flicking a switch.
She said: "She needs reassurance during an episode. It's like she reverts back to being five years old.
"When she's in sleep mode she can be quite moody but she can't help it.
"She stamps her foot if she doesn't get what she wants. It's like having two different kids in the house."
When the first signs of the illness appeared, doctors put it down on Stacey being moody.
Said Ms Richards: "Stacey was tired all the time. She couldn't concentrate at school. She'd come home zonked out and have to sleep the whole weekend. When it came to Monday morning, I couldn't wake her up."
Initially, doctors feared she had a brain tumour, but when the results came back clear, doctors were baffled until a neurologist at Birmingham Children's Hospital eventually diagnosed Stacey with Kleine-Levin syndrome.
Said Ms Richards: "She's gone from a fresh-faced teenager, full of energy to sleeping all the time.
"We can't plan anything because Stacey might be asleep.
"She could go to sleep tonight and wake up next week. We try and laugh and joke about it because it's the only way to get through it."
This article was first published in The New Paper .