Sleep is great. But is too much sleep not great? This is a topic that has been debated, as higher mortality (death) has been linked with too much sleep. What else might long sleep be linked to? We have recently discussed how untreated sleep apnea is linked with poorer cognitive functioning in older adults, meaning shortened sleep is linked with poor cognition. But what about long sleep? In theory, if there is a “normal” sleep amount, then excess on both ends should be bad.
To examine this, 189 cognitive normal older adults (free from Alzheimer’s/dementia) over the age of 75 were tested. These individuals were asked how many hours of sleep they usually have per night, and the research team grouped individuals into different sleep duration groups based on this (short sleepers = less than 7 hours, normal sleepers = 7-9 hours, long sleepers = more than 9 hours). Each participant underwent two standard cognitive tests (the MoCA and the MMSE), each providing a cognitive score. Using these data, the researchers were able to compare the cognitive scores of all of the sleep groups.
Interestingly, a direct comparison found that long sleepers had significantly lower scores than the normal sleepers. However, contrary to what is seen in individuals with sleep apnea, the short sleeping group did not have lower scores than the normal sleepers. When directly comparing long and short sleepers, it was found that short sleepers group were more likely to have higher cognitive scores than long sleepers, even when controlling for factors like age, gender, and depression.
Taken together, these results suggest that the individuals who sleep the longest have poorer cognitive functioning than those who sleep a normal amount and those who sleep less than normal. But can this study say for sure that long sleep is actually causing poor cognition? Definitely not. There are a number of factors that cause a decline cognitive functioning (and some have yet to be discovered). It could be that whatever is causing cognition to decline is also causing an excess of nighttime sleep. Alternatively, individuals might need more sleep because they are waking up many times during the night – possibly even due to untreated and unidentified sleep apnea. In this case, their poor quality (but long) nighttime sleep could indeed be contributing to the decline. Nevertheless, future work is needed to address the directionality of this relationship.
Source: Malek-Ahmadi, M., Kora, K., O’Connor, K., Schofield, S., Coon, D., & Nieri, W. (2015). Longer self-reported sleep duration is associated with decreased performance on the montreal cognitive assessment in older adults. Aging clinical and experimental research, 1-5.