Staring at the ceiling while the clock blinks 3am doesn't only sap energy for the next day. A large, long-running US study of older adults has now linked chronic insomnia to changes inside the brain that set the stage for dementia.

The researchers, from the Mayo Clinic in the US, followed 2,750 people aged 50 and over for an average of five and a half years. Every year the volunteers completed detailed memory tests and many also had brain scans that measured two telltale markers of future cognitive trouble: the buildup of amyloid plaques, and tiny spots of damage in the brain's white matter – known as white-matter hyperintensities.

Participants were classed as having chronic insomnia if their medical records contained at least two insomnia diagnoses a month apart – a definition that captured 16 percent of the sample.

Compared with people who slept soundly, those with chronic insomnia experienced a faster slide in memory and thinking and were 40 percent more likely to develop mild cognitive impairment or dementia over the study period.

When the team looked more closely, they saw that insomnia paired with shorter-than-usual sleep was especially harmful. These poor sleepers already performed as if they were four years older at the first assessment and showed higher levels of both amyloid plaques and white-matter damage.

By contrast, insomniacs who said they were sleeping more than usual, perhaps because their sleep problems had eased, had less white-matter damage than average.