Sleeping nine or more hours each night is associated with worse cognitive performance, an effect that’s stronger among people with symptoms of depression, according to an analysis published in the April 2025 issue of Alzheimer's & Dementia.

Researchers examined sleep duration and cognition in 1,853 adults ages 27 to 85 (average age 50, 57% women) who hadn’t been diagnosed with dementia or suffered a stroke. Participants were categorized into four groups: no depression symptoms and no antidepressant use; depression symptoms and no antidepressant use; antidepressant use and no depression symptoms; and depression symptoms along with antidepressant use.

Those who slept nine or more hours nightly were more likely than participants who slept between six and nine hours each night to score worse on cognitive performance tests evaluating aspects such as memory, visual-spatial skills, and executive function. These associations were stronger in people with symptoms of depression regardless of their antidepressant use. Long sleepers were also more likely to report symptoms of depression. Many experts recommend healthy adults aim to get seven to eight hours of sleep each night. The results of this study don’t prove that sleeping less will prevent or treat depression or other mental health disorders, but the findings may help scientists better understand the connections between sleep and mental health.

(Maureen Salamon is executive editor of Harvard Women’s Health Watch)

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