You’ve heard it before: Too little sleep makes you exhausted, dumb, and sad. But a simple nap will snap you right back into your smart, peppy self, right?
A new JAMA study says nope, because low sleep correlates with lower cognitive function—the permanent kind. That sound you hear is tired people everywhere muttering oh *%$&.
Cohorts of 28,756 people in China and the U.K. reported their nightly sleep hours and underwent cognitive testing, and then repeated the process four years later. All were over age 45. The resulting curve is a U-shape centered around seven hours of sleep: People who get less or more sleep have lower cognitive functioning. And people who reported sleeping around four hours or over 10 hours per night showed not just lower functioning, but also speedier cognitive decline. Agh.
Aside from the obvious horror of losing IQ points to your pillow, this finding is also relevant because the calling card of dementia is cognitive decline, which often appears long before diagnosis. The researchers were keen to identify risk factors, and say that people who do sleep much more or less than seven hours should be evaluated for cognitive functioning.
As for why memory and smarts fade with low sleep, the jury is still out. Though the researchers did control for obvious potential factors like body mass index, education, and smoking, it’s possible that factors other than low sleep might be fueling the fire. Or it might be the sleep. Pro-tip: While the eggheads work that out, play it safe and aim for seven hours, ‘kay?