When Chloe and Evie Church wake for school, they cannot use an alarm clock like other children.
The sisters, aged 11 and nine, have a syndrome that means a loud noise or a sudden shock could be dangerous or even fatal.
The girls from Swansea have Long QT syndrome, a condition thought to affect as many as one in 2,500 people.
The syndrome, which can be passed on through families, causes abnormalities in the heart's electrical system.
"We try not to shout, the girls aren't allowed to have alarm clocks in their bedrooms so we've got to go quite quietly into their rooms in the morning, and wake them gently," said their mother, Samantha.
When the school is planning a fire drill, they have to tell Chloe and her younger sister so they can expect to hear the alarm ringing.
They also have to be supervised by someone who can perform cardiopulmonary resuscitation (CPR) and who is able to take a pulse.
But although the family have had to adapt to the girls' condition, they can still enjoy a relatively normal life.
Last week they were able to go to a Guy Fawkes night party safely because they knew to expect sudden, loud noises from the fireworks.
LONG QT SYNDROME
Causes the heart's electrical system to take longer to recover after heartbeats
Symptons include fainting during exercise or when scared
Around 1 in 2,500 people are affected
Diagnosed easily by examining an electrocardiogram
A sudden noise, or too much exercise is enough to cause a rush or adrenaline or a sharp rise in blood pressure.
This could be fatal, and so the girls will have to take beta-blockers for the rest of their lives.
The girls were diagnosed two months ago with the condition, which also affected their grandmother.
Symptoms normally first appear in childhood, but can occur in babies and during middle age.
Beta-blockers control the condition in about 90% of cases, but some people may need a pacemaker or defibrillator.
"The girls were diagnosed in September and we've adjusted our life to them, slowing down a bit - they know what they can do and what they can't do," said their mother.
Chief executive of Cardiac Risk in the Young (CRY) Alison Cox said over the last 10 years awareness of conditions such as Long QT Syndrome had improved greatly, although there was still a long way to go.
Mrs Cox said CRY's latest research suggested it affected about one in 2,500 people.
It runs a screening programme and Mrs Cox said those most at risk were youngsters where there had been a sudden or explained death in the family.
"The really important thing to understand is with most sudden deaths they do not have any symptoms before hand."
She said that with a programme of medication or the use of internal cardiac defibrillators (ICDs) conditions such as Long QT Syndrome could be controlled.
"Of course some do find major life adjustments difficult - but young people are remarkably robust if they are given the right support," she said.
"Obviously it can be difficult for their parents - but if children feel safe and there is confidence they are being looked after - they have a very good quality of life."
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