medwireNews: People with long sleep durations of more than 9 hours daily have more rapid cognitive decline than those who sleep 6 to 8 hours, researchers report.
“Since sleep duration is potentially modifiable, the relation between sleep duration and cognitive decline might well have practical implications for the primary prevention of [dementia and cognitive] disorders,” they say.
The same relationship was not seen among short sleepers (≤5 hours), however.
Over a 3-year period, 1086 individuals who had a total usual sleep duration, including nighttime sleep and daytime napping, of at least 9 hours experienced an average 0.6-point decrease in Mini-Mental State Examination score (MMSE), from 29.7 to 29.1.
This was significantly greater than the 0.2-point decline experienced by 1331 individuals with a daily sleep time of 6 to 8 hours and the 0.5-point decline in 298 participants with a daily sleep of 5 hours or less. It remained significant after accounting for confounding variables such as age, and the 14 participants whose MMSE changed by more than 15 points. The difference between normal and short sleepers was not significant, however.
The researchers note in the Journal of Psychiatric Research that although the reduction in MMSE was significantly greater in long sleepers, “in absolute terms, it was a modest change.”
They calculate the annual rate of cognitive decline to be 0.2 points/year in long sleepers, significantly greater than the 0.0 points/year for normal sleepers.
It is possible that long sleep duration may be an early symptom of cognitive decline, the team led by Julián Benito-León (University Hospital “12 de Octubre,” Madrid, Spain) comments, or that excessive sleep increases the risk for cognitive decline.
“Currently, however, we know of no plausible physiologic explanation for such a cause-and-effect relationship,” they add.
But whatever the mechanism, “this study provides further evidence that cognitive deficits in sleep disturbances are not static,” the researchers conclude.
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