Australian researchers have found compelling evidence that insomnia may be linked to disruptions in the brain's natural 24-hour rhythm of mental activity, shedding light on why some people struggle to 'switch off' at night.
Published in Sleep Medicine, the study led by the University of South Australia (UniSA) is the first to map how cognitive activity fluctuates across the day in individuals with chronic insomnia, compared to healthy sleepers.
Insomnia affects about 10% of the population, and up to 33% of older adults, with many reporting an overactive or 'racing' mind at night.
While this has long been linked to cognitive hyperarousal, it has remained unclear where these thought patterns stem from.
Researchers examined whether the inability to downregulate mental activity at night - a hallmark of insomnia - reflects underlying circadian rhythm abnormalities.
Under tightly controlled laboratory conditions, 32 older adults were monitored (16 with insomnia and 16 healthy sleepers) over 24 hours of wakeful bedrest.
This approach eliminated environmental and behavioural cues, allowing scientists to isolate the brain's internal rhythms.
Participants remained awake in a dimly-lit room, in bed, with food and activity carefully controlled. They completed hourly checklists, assessing the tone, quality and controllability of their thoughts.
Both healthy sleepers and insomniacs showed clear circadian patterns in mental activity, with peaks in the afternoon and troughs in the early morning.
However, several key differences emerged in the insomnia group.
"Unlike good sleepers, whose cognitive state shifted predictably from daytime problem-solving to nighttime disengagement, those with insomnia failed to downshift as strongly," says lead researcher UniSA Professor Kurt Lushington.
"Their thought patterns stayed more daytime-like in the night-time hours when the brain should be quietening."
Their cognitive peaks were also delayed by around six and a half hours, suggesting that their internal clocks may encourage alert thinking well into the night.
"Sleep is not just about closing your eyes," Prof Lushington says. "It's about the brain disengaging from goal-directed thought and emotional involvement."
"Our study shows that in insomnia, this disengagement is blunted and delayed, likely due to circadian rhythm abnormalities. This means that the brain doesn't receive strong signals to 'power down' at night."
Co-author, UniSA Professor Jill Dorrian, says the findings highlight new treatment possibilities for insomniacs, such as interventions that strengthen circadian rhythms.
"These include timed light exposure and structured daily routines that may restore the natural day-night variation in thought patterns," Prof Dorrian says.
"Practising mindfulness may also help quieten the mind at night."
The researchers say that current treatments often focus on behavioural strategies, but these findings suggest that tailored approaches addressing circadian and cognitive factors could offer a solution.
'Cognitive-affective disengagement: 24-hour rhythm in insomniacs versus healthy good sleepers' is published in Sleep Medicine. DOI: 10.1016/j.sleep.2025.106881. It is authored by researchers from the University of South Australia, Washington State University, and Flinders University.
