A number of studies show that people who sleep approximately 6 1/2 – 7 hours each night live longer and have better cognitive function than those sleeping 8 hours or more.
Daniel F. Kripke, an Emeritus Professor of Psychiatry at the University of California San Diego, analyzed the data on 1.1 million people over a six year period who participated in a cancer study. Published in JAMA Psychiatry the study showed that people who reported they slept 6.5 – 7.4 hours had a lower mortality rate than those who slept for a shorter or longer length of time.
In another study, published in the journal Sleep Medicine in 2011, Dr. Kripke found further evidence that the optimal amount of sleep might be less than the traditional eight hours. The researchers recorded the sleep activity of about 450 elderly women using devices on their wrist for a week. Some 10 years later the researchers found that those who slept fewer than five hours or more than 6.5 hours had a higher mortality.
A study in the journal Frontiers in Human Neuroscience last year used data from users of the cognitive-training website Lumosity. Researchers looked at the self-reported sleeping habits of about 160,000 users who took spatial-memory and matching tests and about 127,000 users who took an arithmetic test. They found that cognitive performance increased as people got more sleep, reaching a peak at seven hours before starting to decline.
A study in the current issue of Journal of Clinical Sleep Medicine tracked five healthy adults who were placed in what the researchers called Stone-Age-like conditions in Germany for more than two months? without electricity, clocks, or running water. Participants fell asleep about two hours earlier and got on average 1.5 hours more sleep than was estimated in their normal lives, the study said. Their average amount of sleep per night was 7.2 hours.