Australian scientists have uncovered strong evidence that chronic insomnia may stem from disruptions in the brain’s internal 24-hour cycle of mental activity. The findings help explain why some people find it so difficult to “switch off” at night, even when they are physically tired.

In a study published in Sleep Medicine, researchers from the University of South Australia (UniSA) tracked how thinking patterns rise and fall across the day in people with long-term insomnia compared with healthy sleepers. This is the first study to chart daily cognitive rhythms in this way.

Insomnia affects roughly 10% of the population and up to one-third of older adults. Many people with the condition describe their minds as overactive or constantly racing at night.

Although this nighttime mental alertness has often been described as cognitive hyperarousal, its underlying cause has remained uncertain. The researchers set out to test whether difficulty calming the mind at night, a defining feature of insomnia, might be linked to abnormalities in circadian rhythms, the body’s internal timekeeping system.