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Resolved to spice up your sexy sack time in 2014? As with most things in life, when it comes to sex, knowledge is power. We've compiled 10 of the most compelling bits of sex research 2013 had to offer in order to add to your sexual knowledge base. Read on for clues to a better sex life -- or to add a few new tricks to your "repertoire." (You're welcome.)

Sexual practice makes perfect. Guys, take note: Women are more likely to have an orgasm with a partner who cares about them (and tries to please them) than with a casual-sexual encounter. 

Having sex four times a week or more may make you healthier, smarter and more successful. Study participants who had sex that often weren't just the stuff of legends -- they also tended to have better strength, endurance, well-being and mental health -- and even an improved mental capacity! But wait, there’s more: This boosted mental and physical state will likely lead to better wages and success at work, researchers say.

Even one night of sleep deprivation can make a guy think a woman wants him. Just like booze, a lack of sleep cons men's brains into perceiving sexual interest that may not be there. So guys, get your Z's if you can't take rejection -- and ladies, give sleepy guys a wide berth if you're not interested.

It seems evolution didn’t mean for women to be monogamous after all. In his 2013 book What A Girl Wants, journalist Daniel Bergner argued that female sexuality may not be the rational, civilized, balancing force it’s often made out to be, and that women’s desires are more animalistic, ravenous and “omnivorous” (meaning diverse) than traditionally thought. He based his book on studies about the nature of women’s sex drives -- and concluded that the idea of female monogamy is nothing more than a “fairy tale.” Discuss.

Rejoice, for "Not tonight -- I have a headache" now means nothing. As it turns out, having sex actually helps cure headaches. So if that's been your go-to "I'm not in the mood" line, you'll have to come up with something else. ("I have groin injury," maybe?)

Emergency contraception has some serious weight limitationsTurns out, heavier women may not be covered by the morning-after pill. In 2013, the FDA said that the European company that makes Plan B One-Step must change its label to warn users that the pill loses its effectiveness if you weigh more than 165 pounds -- and might not work at all if your weight's over 176. This essentially means that Plan B One-Step -- and other contraception that uses the drug levonorgestrel (which blocks the release of eggs before fertilization) -- may be ineffective for the majority of American women, 57 percent of whom are overweight or obese.

Dudes, stash your aprons! Apparently, men who do "women's workdon't get laid as often as guys who take out the trash.

Casual sex is risky business -- especially for your mental health. Casual hookups among young adults can lead to anxiety and depression, a study published in the Journal of Sex Research confirmed. Researchers looked at students from more than 30 universities across the U.S. and discovered that repeated casual-sex encounters triggered severe psychological stress in both genders. 

Besting the Joneses sex-wise will boost your happiness. A University of Colorado Boulder study found that while having sex makes us happy, believing that we’re having more sex than other people makes us even happier. According to the researchers, it's human nature to compare ourselves to others -- but the practice can end up becoming destructive if you don’t keep it in check. (After all, it works both ways: If you believe that you’re having less sex than your peers, you'll feel bad.)

Men and women can’t be platonic friends. No, really. They can’t. In fact, men tend to list romantic attraction as a benefit of their opposite-sex friendships, thereby proving that two people can experience the same relationship in profoundly different ways. (Would you be surprised to learn that most of the men studied believed their platonic female friends wanted to date them -- which often wasn't the case? We wouldn't, either.)

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