There is no “normal” amount of sleep that’s right for everyone
Sleep, as a natural human phenomenon, is still not completely understood despite significant ongoing research. There is no one agreed-upon number experts use as the ideal amount of sleep for everyone to get.
However, determining the right amount of sleep for your own personal system is important because insufficient sleep has a number of negative consequences. A lack of sleep has been shown to negatively affect alertness, memory, problem solving, and overall health, as well as increase the risk of accidents. A 2003 study at the University of Pennsylvania School of Medicine demonstrated that cognitive performance declines with six or fewer hours of sleep. Other studies have shown that a lack of sleep increases cravings for especially fattening foods, and may contribute to obesity and diabetes.
The reverse is also true. Getting good sleep is considered an essential component of overall well being, enabling us to perform at our best during waking hours. Healthcare professionals recognize that quality sleep is a vital component of happiness, and is one of the best forms of preventive medicine.
So instead of a single number, sleep therapists generally recommend a range that is normal for most people. The National Sleep Foundation suggests that seven to nine hours of sleep for adults is optimal. A 2018 study found consistently sleeping 6 to 8 hours per night may be best for cardiovascular health. Other studies have shown six to seven hours increase longevity.
A 2015 cross-cultural study found those living today in some of the world's few remaining preindustrial societies -- meaning hunter gatherer and subsistence farming groups without access to electric lighting or heated and cooled living spaces -- generally spend about 7 to 8.5 hours in bed. These people average about 6 to 7 hours actually sleeping per night, with very little evidence of insomnia or other negative consequences of sleep deprivation.
These observational studies suggest generalized ranges for healthy sleep duration, but on an individual level, each of us is different -- and every one of us has different sleep requirements.
Some people are genetically predisposed to need less sleep than others. A very small percentage, called short sleepers, function normally with 5 or less hours of sleep. Determining the right amount of sleep is also a moving target – it changes as you age.
Normal changes in total sleep and percentage of REM sleep over time
A century ago, before widespread artificial lighting, we slept longer during the winter when the days are short and nights are long, sometimes spending 10, 11, 12 or more hours in bed. There are widespread historical records showing normal sleep a century or more ago during the winter was more biphasic, with two blocks of sleep separated by two or more hours of wakefulness in the middle of the night. Today this is sometimes called the "first sleep, second sleep" phenomenon.
Back then, people might sleep for four solid hours, wake up for two or more hours, then fall back asleep again until morning. The wakeful period in the middle of the night was called “the watch” or watchful sleep period. This was generally described as a period of quietness and peacefulness, almost like a meditative state.