Taking long naps could be a precursor of Alzheimer’s disease, according to a study that tracked the daytime sleeping habits of elderly people. The findings could help resolve the conflicting results of the effects of napping on cognition in older adults, with some previous studies highlighting the benefits of a siesta on mood, alertness and performance on mental tasks. The latest study suggests that an increase over time in naps was linked to a higher chance of developing mild cognitive impairment or Alzheimer’s. The scientists think it is more likely that excessive napping could be an early warning sign, rather than it causing mental decline. “It might be a signal of accelerated ageing,” said Dr Yue Leng, an assistant professor of psychiatry at the University of California San Francisco. “The main takeaway is if you didn’t used to take naps and you notice you’re starting to get more sleepy in the day, it might be a signal of declining cognitive health.” The scientists tracked more than 1,000 people, with an average age of 81, over several years. Each year, the participants wore a watch-like device to track mobility for up to 14 days. Each prolonged period of non-activity from 9am to 7pm was interpreted as a nap. The participants also underwent tests to evaluate cognition each year. At the start of the study 76% of participants had no cognitive impairment, 20% had mild cognitive impairment and 4% had Alzheimer’s disease. For participants who did not develop cognitive impairment, daily daytime napping increased by an average 11 minutes a year. The rate of increase doubled after a diagnosis of mild cognitive impairment to a total of 24 minutes and nearly tripled to a total of 68 minutes after a diagnosis of Alzheimer’s disease, according to the research published in the journal Alzheimer’s and dementia. Overall, participants who napped more than an hour a day had a 40% higher risk of developing Alzheimer’s than those who napped less than an hour a day; and participants who napped at least once a day had a 40% higher risk of developing Alzheimer’s than those who napped less than once a day. Previous research, comparing postmortem brains, showed that those with Alzheimer’s disease were found to have fewer specialist neurons that promote wakefulness. Unusual sleep patterns, insomnia and poor quality night-time sleep are common for people with dementia, but the latest work showed that the link with napping remained even when night-time sleep was taken into account. “This suggested that the role of daytime napping is important itself,” Leng said. The authors said feeling increasingly drowsy during the day could be an early sign that changes were under way in the brain that were the precursors of dementia. Leng said it was not possible to rule out the possibility that napping was causing cognitive problems but there was “no obvious biological mechanism by which taking naps could cause Alzheimer’s”.