The condition will be highlighted as part of a TV series in the UK on medical mysteries - for KLS is certainly that. Little is known about what triggers the sleep disorder and even less about how to cure it.
What is known is that it mainly hits teenagers - the average age it strikes is 16 - and lasts around 13 years, destroying young people's hopes of passing exams, going to university or forging a career.
At the moment, Beth is two-and-a half months into another deep sleep episode. Nothing - not drugs, loud noises, pleading or cajoling - will wake her.
So her life is spent in pyjamas in bed or asleep on the sofa. On the rare occasions she leaves her home in Stockport, Cheshire, to see a doctor, she must be pushed in a wheelchair because she is too tired to walk.
All Janine can do is sit and wait desperately for the "on" switch to flick back in her daughter's head.
"It is like night and day," says Janine, 48. "She might wake up tomorrow and then it's a race against time to live the life she should have had. She rushes off to catch up with her friends and get her hair done. But no one knows when she might fall asleep again."
Beth first started feeling exhausted as a 16-year-old and Janine assumed it was normal teenage torpor.
Then, one evening, Beth fell asleep on the couch and wouldn't wake up. When Janine tried to rouse her, she was horrified when Beth could only babble incoherently in the voice of a five-year-old.
Naturally, her mother assumed the worse: that she had a brain tumour or haemorrhage, and Beth was rushed to hospital. But all tests drew a blank.
Her condition baffled medical staff until a doctor remembered a colleague who had dealt with a similar case.
At the time, Beth had just recovered from tonsillitis, and her medical team suspects the illness was the trigger.
Researchers believe an infection may set off inflammation in the brain in people with a genetic predisposition, and this may damage the thalamus and hypothalamus, the areas responsible for sleep and sensory input.
Since her diagnosis, Beth has been asleep more than she has been awake, sleeping through many of her birthdays and Christmases, as well as holidays. When she wakes up, she has no recollection she was ill or realisation that time has moved on.
Beth's hopes of getting the four A-levels she needed to train to be a child psychologist faded as she was forced to drop out of college.
And Janine, a single mother, had to give up her job to look after her daughter round the clock.
"The most horrible symptom is her confusion," says Janine. "When she wakes for a few hours a day, she does not know where she is and becomes very agitated.
"The toughest year was when her friends finished their A-levels and went off to university, because Beth knew when she woke up that it should have been her, and that hurt her badly. And when she hurts, I hurt."
One person who has stood by Beth, however, is her boyfriend Dan, a 25-year-old primary school teacher she met during an 'awake' phase three years ago.
"He will come round and sit with her nearly every day, talk to her and wait for the girl he fell in love with to come back. When she is awake, they resume their normal adult relationship. He is a good man."
But normally, Beth has no more than two weeks before the syndrome drags her back under again.
Janine says: "Each time, you pray she's had her last episode - and then your heart sinks as you see the signs coming back. Her voice regresses, she starts to find light and noise too much and then she's gone again.
"It breaks my heart to see the best years of her life slipping away."
One of the leading experts on the condition is Dr Guy Leschziner, a consultant neurologist at Guy's and St Thomas' NHS Trust.
He says the number of cases of KLS, named after the doctors who identified the syndrome a century ago, appears to be rising as more young people are diagnosed.
"In the past, milder cases were blamed on teens being lazy and swinging the lead. Otherwise, they were viewed as psychiatric cases or having symptoms of a bipolar disorder."
Beyond sleeping for up to 22 hours a day, the difference with other sleep disorders is that these young people will also show clear personality changes, says Dr Leschziner.
"They feel as if they are in a dream-like state very separate from the world around them. It has a massive impact on their lives. When they wake up and realise what they've missed, they may be depressed and anxious.
"While it's not terminal, young people with KLS can see their lives slipping away in their most formative and important years."
That is how it is for 15-year-old Carew Harris, from Cobham, Surrey, who features in tonight's documentary. A promising footballer, he had been picked to play for a football academy when, after a day spent at a health spa pool four years ago, he was unable to keep his eyes open.
There followed ten days of severe migraines and hallucinations. Carew was sent for an MRI to see if he had a brain tumour. When he recovered, his mother, Danielle, assumed he had just been hit by a virus.
But six weeks later, Carew had another attack of migraines, vomiting and hallucinations after which he was unable to stay awake for more than two hours a day.
Tragically, the closest he now gets to the football pitch are the trophies that line the shelves of the bedroom where he spends most of his life. So far this academic year at his private school, Carew has only been able to attend for just over two weeks.
Danielle, 45, a divorced mother of four, says: "The longest stretch Carew has slept in one go is 40 hours.
"When he is in an episode, you can't wake him up - even if you were standing next to his bed with a brass band playing."
Yet for years, doctors told Danielle to simply give him headache remedies or suggested he was a lazy teenager trying to avoid school, even though he would fall asleep in front of them.
