London, May 6 (IANS) Women who say they are just too tired for sex may be stating the truth, not just fibbing.

 
 They run on a different biological sleep clock to men and need to go to bed earlier because of it, says a new Harvard study.
 
 Women's circadian rhythms run six minutes faster than men's, making them much more likely to be early birds rather than night owls.
 
 Jeanne Duffy, study co-author from Harvard, said: "What we found was that the cycle length of the biological clock in women was shorter on average than it was in men. The average difference was about six minutes."
 
 Researchers report insomnia around 50 percent more frequently than their male counterparts, especially the early morning kind, where they wake up and cannot get back to sleep, the Daily Mail reported.
 
 But women can end up being chronically sleep-deprived because they try to fight against their faster sleep cycles and fit too much into the evenings, before being woken up too early by the light.
 
 The findings were released by researchers at the Harvard-affiliated Brigham and Women's Hospital, based on results produced by 157 people who spent eight weeks in a windowless sleep lab in Boston.
 
 They were isolated from all cues about the time in the outside world, allowing their body clocks to go back to natural circadian rhythms.