Any new parent can tell you, sleep is a stranger, but now, researchers from England's University of Warwick have put a number on it.  They've discovered it takes an agonizing six years for new parents to get back to the amount of sleep they got before baby.

They discovered that mothers of newborns tend to sleep for an hour or less than they used to during the first three months of parenthood, while dads around around 15 minutes in that same period.

Moms gain back around 30 minutes of sleep when the baby is between four and six months, but even between the ages of four to six YEARS, moms still get, on average, 20 minutes less sleep than they used to.

As for dads, between the ages of four to six years, they only lost 15 minutes than in their pre-baby days.

Sleeping issues were more common in first-time parents, and with moms who breastfeed, the study explained.

In a statement that won't be a surprise to sleep-deprived women everywhere -- and one that will likely be brought up as ammo in middle-of-the-night arguments -- study co-author Dr. Sakari Lemola said in a statement: "Women tend to experience more sleep disruption than men after the birth of a child, reflecting that mothers are still more often in the role of the primary caregiver than fathers."