Each year since 1972, the United States General Social Survey has asked men and women: "How happy are you, on a scale of 1 to 3, with 3 being very happy, and 1 being not too happy?" This survey includes a representative sample of men and women of all ages, education levels, income levels, and marital status—1,500 per year for a total of almost 50,000 individuals thus far—and so it gives us a most reliable picture of what's happened to men's and women's happiness over the last few decades.
As you can imagine, a survey this massive generates a multitude of findings, (see the full report by Wharton Professors Betsy Stevenson and Justin Wolfers) but here are the two most important discoveries.
First, since 1972, women's overall level of happiness has dropped, both relative to where they were forty years ago, and relative to men. You find this drop in happiness in women regardless of whether they have kids, how many kids they have, how much money they make, how healthy they are, what job they hold, whether they are married, single or divorced, how old they are, or what race they are. (The one and only exception: African-American women are now slightly happier than they were back in 1972, although they remain less happy than African American men.)