Some news says, "People who take long naps get dementia more easily." Is this true? In the US, 1,401 older people wore a sensor on their wrist.

Scientists followed them for up to 14 years. People with long and frequent naps had a higher risk of dementia. But when dementia got worse, naps also increased. So, long naps may be an early sign, not the cause. Short naps are probably fine.

Headlines sometimes claim that long naps lead to dementia. This research shows why we must be careful about cause and effect.

In a 2022 study, 1,401 older adults in the US wore activity trackers on their wrists, so their naps could be measured objectively. They were followed for up to 14 years. Those who napped longer and more often had a higher risk of later developing Alzheimer's dementia.

But as dementia progressed, daily napping increased more than twice as fast. People with lower thinking ability also tended to nap more a year later. The changes in the brain that cause dementia begin years before symptoms appear. These changes may also make people nap more. So long naps may be an early sign of the disease instead of its cause.

Headlines warning that long naps raise the risk of dementia run into an old problem in health research, which is telling association from causation. In a 2022 cohort study, 1,401 older Americans wore activity monitors on their wrists so that daytime napping could be measured objectively, and they were followed for up to 14 years. Longer and more frequent naps went with a higher later risk of Alzheimer's dementia.

But the relationship ran both ways. After diagnosis, the yearly increase in napping more than doubled, and poorer cognition predicted more napping a year later. The brain changes behind dementia begin years before symptoms. They may disrupt the systems that keep us awake in the daytime, which would make heavy napping an early sign of the disease instead of a cause.

Researchers call this reverse causation. A 2023 British study of about 830,000 women backed the idea, since the link weakened substantially as follow-up lengthened. Both were observational studies, and wrist monitors can mistake quiet rest for napping. Nap length may matter too. Japanese research has linked short naps to slower cognitive decline.