No one's really sure. The more we learn about sleep, the more confusing it becomes.
Adults need between six-and-a-half and eight hours of sleep each night. Why the long spread? Everyone has different individual needs, and there isn't any clear connection between this need and any outside factors.
While sleep deprivation has some obvious problems, there is some argument that sleeping too much may be bad for you, but it's currently unclear whether excessive sleep causes health problems and depression, or whether bad health and depression cause people to sleep more. No matter which way this chicken or egg problem is resolved, there's still a direct correlation between having these medical problems and regularly sleeping more than nine hours per night.
The extra time you need to sleep to catch up on previously missed sleep is called a sleep debt. It was originally thought that you could sleep half as long as the time you missed and being fully rested. For example, if you missed four hours of sleep, the next night you would need to sleep an additional two hours. It is clear that one cannot adjust to a lack of sleep: The sleep debt remains no matter how long you put off getting additional rest. Instead, you may come to accept the lack of mental focus that comes with sleep deprivation.
Current research into determining sleep debt is focusing on two measurements: the Epworth Sleepiness Scale and the amount of amylase in saliva. Both are thought to show a real indication of sleep needs, and may eventually lead to sleep standards for people in schedule-intensive jobs, like doctors and pilots.
So, how much sleep do you need? Enough to feel rested. How much is that? It depends on your specific needs.
Posted 4543 day ago