A new prospective cohort study published last week in the journal SLEEP suggests that sleeping for 7 or more hours every night might not be all that it’s cracked up to be after all – at least when it comes to cognitive ability, gray matter volume, and white matter microstructure.
The study was performed on 613 total participants who self-reported their sleep duration at five time points between 1985 and 2013, and were subjected to a battery of cognitive tests and magnetic resonance imaging at a single time point between 2012 and 2016.
Sleep duration was assessed by way of a questionnaire, resulting in the following breakdown: 29 participants reported typically sleeping 5.4 hours per night, 228 – 6.2 hours per night, 278 – 7 hours per night, and 78 – 7.9 hours per night.
At the outset of the study, the authors speculated that following general sleep recommendations provided by most national and international organisations would be superior in terms of both cognitive performance and gray matter volume.
After analysing the results, however, the researchers found that sleep duration, even when it’s trending towards the extremes, does not seem to have any real effect on the measures selected for the study.
“In contrast to our hypotheses, our results did not show any differences in cognitive measures, gray matter volume, or white matter microstructure between different sleep trajectory groups”, wrote the authors in their paper.
One possible explanation for the lack of any significant relationship between sleep duration and cognitive ability is the “limited number of participants reporting extremes in sleep duration” within the sample (between 92% and 96% of participants reported 6, 7, or 8 hours sleep per night).
Furthermore, significant “group differences for cognitive and MRI measures may only become apparent with larger samples of more extreme sleep durations, groups that have often been the focus of cognitive studies to date”.
An alternative explanation, favoured by the authors themselves, would be that sleep duration is inseparable from sleep quality – an idea supported by a paper published by the group back in 2017.
“If replicated, such null results could challenge the suitability of current sleep guidelines on cognitive outcomes and open doors to new directions in research regarding the appropriateness of considering sleep duration and quality together”.
Source: Jennifer Zitser, Melis Anatürk, Enikő Zsoldos, Abda Mahmood, Nicola Filippini, Sana Suri, Yue Leng, Kristine Yaffe, Archana Singh-Manoux, Mika Kivimaki, Klaus Ebmeier, Claire Sexton, Sleep duration over 28 years, cognition, gray matter volume, and white matter microstructure: a prospective cohort study, Sleep, , zsz290, https://doi.org/10.1093/sleep/zsz290