You already know that a lack of sleep can leave you feeling cranky and sluggish the next morning, but a new study from the University of California at Berkeley found that skimping on shut-eye may make you more selfish, too—and that’s bad news when it comes to a.m. interactions with your significant other.“Our research found that a poor night’s sleep causes people to prioritize their own needs over others,” explains Amie Gordon, a U.C. Berkeley psychologist and lead investigator of the study. “When we tracked study participants over two weeks, we found they reported more gratitude toward their partners on the nights they slept well and less on the nights they slept poorly.”Translation: Less zzz’s can make you less appreciative and empathetic.So when your partner takes out the garbage, you’re less likely to say “thank you” (Do you want a medal just because you took out the trash?) or when he complains he’s exhausted, you’re less likely to sympathetically rub his back and more likely to complain about your own fatigue (You think you’re tired? I was up two hours before you!), leaving you both in a crabby mood. And all it takes is one night of tossing and turning to make you feel less appreciative of your partner.Luckily, being aware that this can happen could help diffuse tension before it begins, says Gordon. “Knowing you’re already exhausted can help you stop thinking something is wrong with your relationship or your partner,” says Gordon.To combat the lack of gratitude brought on by too little shut-eye, try making “thank you” an essential phrase in the morning. On the days you’re exhausted, it won’t come naturally, so turning it into a game—you have to say thank you about something before you leave the house—can ensure it’ll actually happen.Finally, a good exit strategy if you find yourself in the middle of a “you never remember to put away the milk” argument is to try to find something funny about what’s going on. “Studies have found that humor can help circumvent conflict and break a downward spiral of negativity,” notes Gordon. But while these fixes can help make a tired morning a bit better, making sleep a priority is the easiest way to stop these negative feelings before they even happen. “Logging a good night’s sleep will naturally make you feel more grateful toward your partner,” says Gordon.QUIZ: What’s Your Relationship Style?