Imagine being so tired you literally cannot keep your eyes open when you drive.
That's something Sydney nurse Fran Boyer experienced for two years until a car accident served as a wake-up call.
"I fell asleep less than a kilometre from home. I woke up and the car had crashed into a railing. I was sound asleep when it happened," she said.
It wasn't the first time she had dozed off behind the wheel.
"I fell asleep at the traffic lights while driving a few times. People had to toot their horns to wake me up," she said.
"I thought it was just tiredness, just getting older."
She never suspected she had a common sleep disorder where parts of her brain were shutting down due to fatigue.
Ms Boyer had also been struggling to study, so she knew something was not right.
"I thought I had early onset dementia or I was stupid. Now I realise my brain wasn't working properly," she said.
After the car accident, she went straight to her GP, who recommended she take part in a sleep study.
Doctors found the 60-year-old had obstructive sleep apnoea.
Disrupted sleep, hundreds of times a night
As many as one-quarter of women and half of men over 50 have sleep apnoea, though many are not diagnosed.
Symptoms include poor-quality sleep with loud snoring, reduced breathing and gasping for air, daytime sleepiness, dry mouth and headaches on waking, poor concentration, irritability, depression and anxiety.
In obstructive sleep apnoea, the throat collapses while you sleep so that little or no air gets into the lungs.
It stops you breathing long enough for the brain to notice and send a warning, so you wake up and your throat muscles start working again.
This process of disrupted sleep can happen hundreds of times a night.
That was the case for Ms Boyer, who stopped breathing 10-15 times an hour while asleep.
She now sleeps with a continuous positive airway pressure (CPAP) machine to keep her airways open.
Ms Boyer is part of an ongoing study being conducted at Sydney's Woolcock Institute, which is uncovering new insights into the brains of people with sleep apnoea.
Lead researcher Angela D'Rozario said when you use certain parts of the brain during the day, you need deep sleep to preserve and restore those parts of the brain.
"People with sleep apnoea don't get the restorative deep sleep they need, which helps regenerate the parts of the brain needed for driving," Dr D'Rozario said.
Her team is studying people with sleep apnoea, putting them through challenging driving stimulations before fitting them with a cap with 250 electrodes to monitor brain activity.
The team is investigating why people with sleep apnoea can have significant brain fog and difficulty concentrating on tasks like driving.
'The lights are on but nobody's home'
Dr D'Rozario believes the brain cannot cope with the lack of sleep so it makes up for that deficit during the day.
"We think the neurons are going offline and behaving like they are asleep," Dr D'Rozario said.
"You wouldn't know these people are asleep as their eyes are open. The lights are on but nobody's home."
Having people driving around with parts of their brain asleep poses a serious risk to road safety.
It is estimated more than 20 per cent of road accidents are due to fatigue. Car accidents caused by tiredness are also more likely to be fatal than those caused by other things.
Lynne Bilston, the senior principal research scientist at Neuroscience Research Australia, said it was important to understand more about what was happening in the brains of people with sleep apnoea.
"Obstructive sleep apnoea is associated with a 2.4-times increased risk of motor vehicle crashes, which has been compared to driving while over the blood alcohol limit," Professor Bilston said.
"The risk of car crashes is greatly reduced when people are compliant with an effective treatment for sleep apnoea."
Since being treated for the condition, Ms Boyer has been feeling a lot better.
"I feel like I have gained 10 years," she said.
"When I wake up, I have energy and I actually want to get out of bed. The brain fog has definitely lifted."
The Woolcock Institute is hoping to recruit more people with sleep apnoea to continue their research.
