A few weeks ago, a study landed in the journal Brain, finding that participants who routinely logged roughly six to eight hours of quality sleep appeared to stave off cognitive decline.
Our study suggests that there is a middle range, or 'sweet spot,' for total sleep time where cognitive performance was stable over time," said study co-author Dr. Brendan Lucey in a statement.
Lucey is an associate professor of neurology and section head of the Washington University Sleep Medicine Center in St. Louis. [CNN Health Keep your brain sharp by finding your sleep 'sweet spot,' study says]
To further complement these findings, a recent study published in JAMA Neurology found that “Elderly people who slept fewer than six hours a night had more beta-amyloid in their brain than those who slept between seven and eight hours. Beta-amyloid is a hallmark sign of Alzheimer's disease.”
In a nutshell, these studies seem to point towards two basic facts when it comes to brain health:
Subjectively, haven’t we all found this to be the case at some point in our lives? On those occasions when you logged a low amount of sleep, your cognitive performance struggled. However, counterintuitively, on those days when you logged far more sleep beyond your usual nightly numbers, then too, you experienced that foggy-headed mental malaise that can come with oversleeping.
There does appear to be an emerging bell curve of desirable sleep times, with observable ill effects occurring not only with cognitive performance but overall biometrics in general.
When you tip the scales towards hypersomnia—excessive daytime sleepiness or excessive time spent sleeping—we tend to see negative shifts in nighttime heart rate, respiratory rate, HRV, and body temperature. All are critical indicators of poor overall recovery. Conversely, when dealing with insomnia—difficulty falling asleep or staying asleep—you can often see similar negative correlations with nocturnal heart rate, respiratory rate, HRV, and body temperature.
While there are certainly cases when people are exclusively short sleepers or long sleepers, from a behavioral perspective, often we see repeated swings to opposing sides of the sleep time pendulum, night after night.
You can think of it as the sleep duration roller coaster. The typical ride often begins with a night of short sleep, followed by massive overcorrections toward oversleeping on a subsequent night. That hypersleep results in difficulty maintaining a usual sleep bedtime-waketime and another night of short sleep results. Unless interrupted, this thrill ride can play out repeatedly.
Rather than this up and down behavior, we could take the findings of these two studies to point to an age-old truth:
Consistency is king when it comes to sleep.
This consistency can often make it so much easier to hit our sleep duration goals, night after night.
Routinely maintaining our sleep time “sweet spot”—between 6 to 8 hours and avoiding riding the high and low sleep rollercoaster—could be crucial to brain health and perhaps improved health in general.
 Brendan P Lucey, Julie Wisch, Anna H Boerwinkle, Eric C Landsness, Cristina D Toedebusch, Jennifer S McLeland, Omar H Butt, Jason Hassenstab, John C Morris, Beau M Ances, David M Holtzman, Sleep and longitudinal cognitive performance in preclinical and early symptomatic Alzheimer’s disease, Brain, Volume 144, Issue 9, September 2021, Pages 2852–2862, View Study
 Winer JR, Deters KD, Kennedy G, et al. Association of Short and Long Sleep Duration With Amyloid-? Burden and Cognition in Aging. JAMA Neurol. 2021;78(10):1187–1196. doi:10.1001/jamaneurol.2021.2876