THIS should explain why your wife or girlfriend always wants to wake up early on Saturday to do brunch and take on the day . . . when all you want to do is keep sleeping.

--A study at Brigham and Women's Hospital in Massachusetts found that the average woman's internal clock runs about six minutes faster than the average man's.

--A six-minute difference doesn't seem like much, but the researchers found it makes a huge difference.

--Women are much more likely to identify themselves as "morning people" . . . have an easier time getting out of bed . . . and have more trouble staying up late when they're tired. It also leads to more insomnia in women.

--The researchers say that if you are a woman who has trouble with insomnia or constant fatigue, one strategy is getting shades that make your bedroom go completely dark.

--Your internal clock is tuned to need darkness earlier . . . so by blocking out all of the light, it makes it easier for you to go to sleep.