Sleeping too much can harm your brain function, according to a study which shows that those who sleep less or more than seven to eight hours a night have worse cognitive performance.
Researchers from Western University in Canada said that for the world's largest sleep study, launched in June last year, over 40,000 people from around the world participated in the online scientific investigation.
The study, published in the journal Sleep, included an in-depth questionnaire and a series of cognitive performance activities.
"We really wanted to capture the sleeping habits of people around the entire globe. Obviously, there have been many smaller sleep studies of people in laboratories but we wanted to find out what sleep is like in the real world," said Adrian Owen, from Western University.
About half of all participants reported typically sleeping less than 6.3 hours per night -- an hour less than the study's recommended amount.
One startling revelation was that most participants who slept four hours or less performed as if they were almost nine years older.
Another surprising discovery was that sleep affected all adults equally. The amount of sleep associated with highly functional cognitive behaviour was the same for everyone (7 to 8 hours), regardless of age.
Also, the impairment associated with too little, or too much, sleep did not depend on the age of the participants.
"We found that the optimum amount of sleep to keep your brain performing its best is 7 to 8 hours every night and that corresponds to what the doctors will tell you to need to keep your body in tip-top shape, as well," said Conor Wild, a research associate at Western University.
"We also found that people that slept more than that amount were equally impaired as those who slept too little," said Wild.
Participants' reasoning and verbal abilities were two of the actions most strongly affected by sleep while short-term memory performance was relatively unaffected.
This is different than findings in most scientific studies of complete sleep deprivation and suggests that not getting enough sleep for an extended period affects your brain differently than staying up all night.
On the positive side, there was some evidence that even a single night's sleep can affect a person's ability to think. Participants who slept more than usual the night before participating in the study performed better than those who slept their usual amount or less.
(With inputs from agencies.)