In 1983 D.L. Wingard of the University of California and L.F. Berkman of the University of the Witwatersrand published a paper tracing the effect of sleep duration on premature death. The study was based on a 1956 poll conducted among 6,928 Canadians, whose mortality was tracked 9 years later. It turned out that the mortality rate among men who slept 7 or 8 hours a night was 1.7 times lower than those who slept more or less. For women, this discrepancy was 1.6 times lower.
T.E. Gildner et al. of Dartmouth College published a 2014 paper on the effect of sleep duration on cognitive skills. The analysis was conducted based on a survey of 34,203 older people (50 years old and up) from six countries. All of these people were asked to report how many hours they sleep a day, and each of them passed five cognitive tests. A significant correlation was found between sleep duration and mental performance, with the best results recorded for participants who slept 6 to 9 hours a day.
In 2015, J. Wu et al. of Zhejiang University published a paper examining the habits of 66,817 Chinese teenagers. The researchers found a clear correlation between obesity and sleep duration. The correlation was U-shaped: the curve bottomed out around 7–8 hours a night, while the likelihood of problems with excess weight greatly increased with even minor deviations to either side. The number of obese teens almost doubled for every extra/missing hour of sleep.
M.G. Lima et al. of the University of Campinas published a 2018 paper on the effects of sleep excess and deprivation on various factors, including health and happiness. 1,969 people over 20 y.o. took part in the survey. It was found that sleeping less than 7 and more than 8 hours not only increased the number of chronic diseases but also decreased the level of life satisfaction. Moreover, while the likelihood of chronic diseases increased by approximately one and a half times on average for both groups, people suffering from sleep deprivation were 2.4 times more likely to be unhappy, and those indulging in excessive sleep were twice as likely to be unhappy.