Longer and more frequent naps were linked with a higher risk of Alzheimer's disease, new research reports.
A study suggests that an increase in naps - and their length - in older adults was connected to mental decline in the future and vice versa.
That means being diagnosed with Alzheimer's was found to increase the number of naps people take.
Increased sleep during the day may be a sign that the disease has gotten worse, the study reported.
Researchers named it a "vicious cycle" and called for closer monitoring of 24-hour sleep patterns in older people.
Alzheimer's is a condition in which memory and thinking skills continuously decline.
There is no cure and it is the most common cause of dementia.
"The vicious cycle we observed between daytime sleep and Alzheimer's disease offers a basis for better understanding the role of sleep in the development and progression of Alzheimer's disease in older adults," said lead author Dr Peng Li.
Published in the journal Alzheimer's & Dementia, the study aimed to find the connection between daytime napping and the disease.
Its findings could clarify conflicting results about the effects of napping on cognition in older adults.
Some studies show that dozing off is good for cognitive performance, mood, and alertness.
This study found that excessive napping - meaning longer or more frequent snoozing - was linked to worse cognition a year later.
At the same time, worse cognition was correlated with more excessive naps in the following year.
Scientists tracked the sleep patterns of over 1,000 people over the course of several years.
Participants, whose average age was 81, wore watch-like devices for a period up to 14 days and underwent cognitive tests yearly.
People with no cognitive impairment nodded off an extra 11 minutes a year.
Naps doubled after being diagnosed with mild cognitive impairment, increasing daily daytime napping by 24 minutes.
Remarkably, time slept napping tripled to 68 minutes after an Alzheimer's diagnosis.
"Our hope is to draw more attention to daytime sleep patterns and the importance of patients noting if their sleep schedule is changing over time," said co-senior author Dr Kun Hu.
"Sleep changes are critical in shaping the internal changes in the brain related to the circadian clocks, cognitive decline and the risk of dementia."
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