They Get No Sleep
No rest for the weary applies to all parents, but single moms and dads carry a greater burden. According to the Centers for Disease Control, single parents (especially women) are more likely to have shorter sleep duration.

It's no surprise. Single parents are up before the kids, at work all day, and then come home to make dinner and help with homework. Any alone time after the children are in bed comes at the expense of much-needed z's.