New research reveals that sleeping more than nine hours a night may harm cognitive performance—especially for people with depression—challenging assumptions about what counts as healthy rest.
Key Points at a Glance
- Sleeping 9+ hours linked to lower memory, focus, and problem-solving ability
- Effects strongest in people with depressive symptoms, regardless of medication
- Study draws on nearly 2,000 participants from the Framingham Heart Study
- Long sleep, not short sleep, correlated with worse cognitive function
- Findings highlight need to balance sleep and mental health strategies
Sleep has long been seen as medicine for the mind—essential for memory, mood, and brain health. But a new study by researchers at The University of Texas Health Science Center at San Antonio flips that notion on its head: when it comes to sleep, more isn’t always better. In fact, for people struggling with depression, too much sleep may actually impair cognitive performance.
Published in Alzheimer’s & Dementia: The Journal of the Alzheimer’s Association, the study analyzed sleep habits and mental performance in 1,853 adults aged 27 to 85, all participants in the well-known Framingham Heart Study. None had a history of stroke or dementia. The research team examined whether depression altered the relationship between how long people sleep and how well they think.
What they found is both surprising and significant: long sleep—defined as nine hours or more per night—was consistently associated with reduced performance on tasks involving memory, spatial reasoning, and executive function. This effect was most pronounced in individuals with symptoms of depression, whether or not they were taking antidepressants.
“Sleep may be a modifiable risk for cognitive decline in people with depression,” said Vanessa Young, MS, the study’s first author and a clinical research project manager at UT Health San Antonio’s Glenn Biggs Institute for Alzheimer’s and Neurodegenerative Diseases.
Importantly, the study did not find a similar impact for short sleep. It wasn’t too little sleep causing cognitive strain—it was too much.
According to Sudha Seshadri, senior author and founding director of the Biggs Institute, these findings add crucial nuance to what we understand about brain health. “Long but not short sleep duration was associated with poorer global cognition and specific cognitive abilities like memory, visuospatial skills and executive functions,” she said. The presence of depressive symptoms only amplified the cognitive consequences.
This challenges the traditional view that getting “as much sleep as possible” is ideal for mental health. The Global Council on Brain Health currently recommends 7 to 8 hours of sleep per night for adults—a range that seems validated by the study’s findings.
The research is also timely in light of growing awareness about sleep and its relationship to neurodegenerative conditions such as Alzheimer’s disease. While poor sleep has often been associated with cognitive decline, the new findings suggest that excessive sleep—especially in those with mental health concerns—should also be examined more closely.
Depression itself is already a known risk factor for cognitive decline and dementia, and it’s intricately tied to sleep disruptions. Around 90% of people with depression report some form of sleep problem. But by isolating long sleep as a specific variable, the UT Health San Antonio team brings sharper clarity to the connection between mood, rest, and cognition.
The research team divided participants into four categories: those with no depressive symptoms and no antidepressants; those with depressive symptoms but no medication; those on antidepressants but with no current symptoms; and those with both symptoms and medication. In all but the third group, long sleep was linked to worse cognition—suggesting that antidepressants alone aren’t a protective factor against the cognitive costs of oversleeping.
As with many sleep studies, the findings don’t establish a definitive cause-and-effect relationship, but they do raise important questions. Does long sleep exacerbate depression’s impact on the brain? Or does worsening brain function prompt people to sleep longer as a coping mechanism? The researchers note that future studies using longitudinal data and multi-modal approaches will be needed to untangle these questions.
For now, the takeaway is both cautionary and empowering: oversleeping may be a silent contributor to mental fog, especially in people already facing depression. Adjusting sleep habits, with the guidance of healthcare providers, could offer a new lever for improving cognitive health.
Source: University of Texas Health Science Center at San Antonio
