American scientists confirmed that a mutation in one of the genes allows some people to regain their activity and get enough rest without the need for long hours of sleep.   Most people need at least six hours of continuous sleep to restore their activity and ability to focus, but US researchers discovered the existence of a “genetic variable” in some people that makes little sleep enough to restore their activity and carry out their daily duties.   According to the study conducted by researchers at the Children’s Hospital in Philadelphia on 100 pairs of twins, and its results were…