 Conventional wisdom has it that over the last 50 years sleep duration has declined in parallel with the increasing prevalence of obesity, suggesting that an epidemic of sleep loss is associated with the epidemic of weight gain. Now we have triple-digit streaming, TV channels, smartphones, and tablets to keep us entertained well into the night. The hurry and excitement of modern life is quite correctly held to be responsible for much of the insomnia. I did one medical journal editorial, but that was an editorial published in 1894. Are we really sleeping that much less? It's 1905. Sleep duration in Shultern and Adolescents has declined by a little over an hour a night. However, child labor was an outlaw until 1938, though, and so part of the explanation may be due to the exhaustion of sweating it out in the mines, farms, and factories in the early part of the last century. Since 1970, youth sleep duration has only declined about 15 minutes per night, and it's not clear sleep duration in adults has changed much at all. Based on 168 studies of objective measurements of sleep duration, instead of just self-report, sleep duration in adults. Objective total sleep time hasn't changed much since 1960. Since 2003, average sleep duration in the United States may have even gone up. And of course, just because we don't have evidence that there's been a growing epidemic of sleep deprivation, that doesn't necessarily mean we're getting enough sleep. Maybe we weren't getting enough sleep 50 years ago either, or since Edison's light bulbs, or since candles were invented 5,000 years ago. How might we determine the optimal sleep duration? One way would be to study millions of people and see how many hours a night is associated with the longest lifespan. Sleep is a great mystery. A trait shared across animal species, sleep must be of vital importance to survive natural selection pressures to eliminate such a vulnerable state. Indeed, cringe-worthy experiments have shown that keeping animals awake long enough is fatal within 11 to 32 days. One of the functions of sleep that has been elucidated in recent years is the clearance of toxic waste substances that build up during the day through a newly discovered drainage system in the brain. This could help explain why those who routinely get less than 7 hours of sleep at night may be at an increased risk of developing cognitive disorders such as dementia. Even a single all-nighter can cause a significant increase in beta-amyloid accumulation in critical brain areas, a gummy substance implicated in the development of Alzheimer's disease. The lowest risk for developing cognitive impairment was found for those getting 7 to 8 hours of sleep at night, which is the same optimal range found for diabetes risk based on 36 studies following more than a million people. The increased risk associated with getting only 6 hours a night compared to 7 or 8 is comparable to the increase in diabetes risk linked to physical inactivity. For death from all causes combined, there's been more than 50 studies following hundreds of thousands of people for up to 34 years, sleeping too short, and too long are both associated with cutting one's life short with the apparent sweet spot at 7 hours a night. 7 hours may seem short, but that may actually be what's natural for our species. Scientists studied three isolated pre-industrial societies across two continents and found a surprising uniformity. Despite no electric lighting or gadgets, they stayed up until about three hours after sunset and then typically rose before dawn, accumulating about a solid 6 and a half hours of sleep out of about 7 and a half hours in, quote unquote, bed. A mechanism by which excess sleep might be harmful remains elusive, and so the association between increased risk of death and disease and sleeping 9 or more hours a night has been largely dismissed as implausible. Maybe it's reverse causation, sickness leading to more time in bed instead of vice versa, or confounding factors such as employment status. After all, who gets to sleep in? Those without a job to get to. However, there is experimental evidence showing negative health effects from insufficient sleep, so in terms of sleeping in the sweet spot, aim for at least 7 hours of regular sleep a day.