Original sentence: A large US study linked life spans with sleep amount and found that those who sleep 4 to 7 hours live the longest, with those sleeping more than 9 or less than 4 hours living shorter lives.It is important to mention this does not apply to children and adolescents, particularly children who require as much as 13 hours a night.The National Sleep Foundation however maintains that 8 hours of sleep is optimal, claiming improved performance in tests, reduced risk of accidents and a better immune system
Edited sentence: Although it is important to mention this does not apply to children and adolescents, particularly children who require as much as 13 hours a night.People who regulary sleep between 4 and 7 hours live longer than people sleeping more or less than this, with serious differences between life span when people sleep less than four hours or more than 9.The most detailed study on sleep to date revealed the optimum time spent sleeping to be 7 hours