The general rule of thumb for what counts as a full night's sleep has been eight hours for as long as anyone can remember. Despite this, most of us are happy to get by on less. After all, what difference does an extra hour or two really make? As it turns out, quite a lot. According to science, sleeping even seven hours a night will slowly add up to a costly sleep debt.
Two studies, one at the University of Pennsylvania and another at the Walter Reed Research Institute, tested dozens of sleepers in a bid to find out how much sleep the average human needs. The studies lasted about four years and posted their results back in 2003.
Despite this, most people still operate under the old assumption that five or six hours of nightly sleep is enough to operate indefinitely, without any consequences. While it's true that a small percentage of people can actually pull it off, the number that most of us need is a rock-solid eight hours — anything less and we begin to suffer major losses in everything from attention span to reaction time.
The participants in the study were tested in a controlled laboratory setting for a period of two weeks. Those who slept eight or nine hours didn't show any signs of slowing down when tested, but the four and six hour groups were found to be impaired to the point of being "the cognitive equivalent of being legally drunk" at the end of the two-week period. Sleeping in on the weekends won't necessarily make it better, either. We just need to sleep more.
If you're having trouble meeting your eight hour quota, here are some tips for sleeping better.
This article has been revised and updated from its original publication date.