The authors had the hypothesis that people who consistently slept at least seven hours per night would have improved cognition as well as improved grey matter and white matter structure than those people who consistently slept less than the guidelines recommend.
The results, however, were quite surprising.
In general, they found that for most people sleep duration remained relatively stable over 28 years. About 12.7 percent of the participants slept eight hours, about 45.4 percent slept seven hours, 37.2 percent slept six hours and 4.7 percent slept five hours each night.
Remarkably, there were absolutely no differences in cognition or any of the brain parameters between the different sleep duration groups. Thus, sleeping six or even five hours each night was not associated with impaired cognition or any negative effects on the brain. The authors suggested that it might be more meaningful to also analyze sleep quality when assessing the impact of sleep on cognition and brain structure. Moreover, this suggested that current sleep duration guidelines might need to be redesigned if further studies support their results.