LONDON, June 23 (Xinhua) -- Teenagers who are used to staying up late and getting up late now have good reasons -- they have a later body clock, according to U.S. scientists.
A study conducted by Harvard T.H. Chan School of Public Health showed that the optimum bedtime for 17 and 18-year-olds is 12:30 a.m., while they should wake up at 8:30 a.m., British paper the Daily Mail reported Wednesday on its website.
Teenagers' later body clock is compared to that of 60-year-olds, who are larks and typically go to bed at 11 p.m. and get up at 7 a.m., the report said.
It is unknown why teenagers' sleeping patterns make them sleep later, only to start getting up earlier at around the age of 19, it said.
But the study said blue light from smartphones and tablets may be affecting human circadian rhythms, while some experts say their habit of staying up late might also be changing their body clocks.
The study also found that men also sleep later than women up until the age of 40, perhaps because women's lives are changing their sleeping patterns through less exposure to natural light as they take on the bulk of housework and childcare.
The findings are based on data from almost 54,000 people from 2003 to 2014.
