Feel like you're losing your marbles when you haven't had enough sleep? Well, you might not be too far off the mark.
Research has previously found that we can never fully repay a sleep debt.
A new study has found that chronic lack of sleep may also lead to irreversible loss of brain cells.
Published in The Journal of Neuroscience, the researchers revealed that extended wakefulness is linked to injury to, and loss of, locus coeruleus (LC) neurons that are essential for alertness and optimal cognition.
"In general, we've always assumed full recovery of cognition following short- and long-term sleep loss," said lead researcher Sigrid Veasey, from the University of Pennsylvania. "But some of the research in humans has shown that attention span and several other aspects of cognition may not normalise even with three days of recovery sleep, raising the question of lasting injury in the brain.
"We wanted to figure out exactly whether chronic sleep loss injures neurons, whether the injury is reversible, and which neurons are involved."
To determine the damage to the neurons, the researchers put mice on the same typically truncated sleep pattern as shift workers; they had periods of resting normally, followed by periods of short and extended wakefulness.
After just several days of shift worker sleep patterns, there was increased cell death, and the mice lost 25 per cent of the LC neurons.
"This is the first report that sleep loss can actually result in a loss of neurons," Veasey noted.
The findings are not surprising, says Dr Angela D'Rozario, from the Woolcock Institute of Medical Research.
"There's gradually increasing evidence that disruption to sleep results in neural injury that we can't fully reverse," says D'Rozario, a post-doctorate research fellow, specialising in sleep and circadian rhythms. "For all its adaptiveness, the brain is fairly sensitive as well."
While it is clear that exposure to sleep deprivation has a detrimental effect on us mentally, physically and emotionally, the tipping point varies among individuals.
Not everyone requires the same amount of sleep and recent research found that, generally speaking, women need more than men. Still, around half of Australians say they are "exhausted" and sleep-deprived.
"What is our capacity to compensate for sleep loss," D'Rozario asks. "People respond so differently. Some people cope fairly well with sleep loss whereas others really struggle... But we don't know why."
Understanding this is the "holy grail" of sleep study, says D'Rozario.
The next step is working out whether the latest research translates to humans and determining what durations of wakefulness place individuals at risk of neural injury.
The Penn University team plan to examine these effects in shift workers. They are also planning autopsy studies of shift workers for evidence of increased LC neuron loss and signs of neurodegenerative disorders such as Alzheimer's and Parkinson's.
"If we can show that we can protect the cells and wakefulness, then we're launched in the direction of a promising therapeutic target for millions of shift workers," Veasey says.
While many of the mysteries around sleep remain elusive as a dream, there is one thing those in the field of sleep study know for sure.
"We are all too ready to swap sleep for social activities or work activities," D'Rozario says. "But we need to recognise the risks and that sleep is really important."
- Sydney Morning Herald