Sleep versus cuddling: science is finally weighing in on what happens after sex.According to a recent study by evolutionary psychologists at the University of Michigan and Albright College in Pennsylvania, the tendency to fall asleep first after sex is associated with greater partner desire for bonding and affection."The more one's partner was likely to fall asleep after sex, the stronger the desire for bonding," explains Daniel Kruger, research fellow at the University of Michigan, and lead author of the study.The study, published in December in the Journal of Social, Evolutionary, and Cultural Psychology, examined 456 participants, who completed anonymous online surveys assessing experiences and desires with one's partner after sex. Participants then indicated "who falls asleep after sex?" and "who falls asleep first when going to bed not after sex?."Participants whose partners nodded off immediately after sex had stronger desires for post-coital cuddling and chatting."Falling asleep before one's partner may be a non-conscious way to foreclose on any commitment conversation after sex," says co-author Susan Hughes, associate professor of psychology at Albright College in Reading, Pa.The study also looked at who were more likely -- men or women -- to fall asleep first.Despite the common stereotype, the researchers did not find it more common for men to fall asleep first after sex. Women, however, were more likely to fall asleep first when sex hadn't taken place."Perhaps men stay awake longer as an artifact of mate guarding -- making sure the woman doesn't leave them for another partner," says Hughes. "Men may also stay awake longer in an attempt to entice their partner into having sex."Research on post-coital behaviors are few, the study authors say. "The vast majority of the research on the evolutionary psychology of human reproduction focuses on what's before and leading up to sexual intercourse," says Hughes. "But reproductive strategies don't end with intercourse; they may influence specific behaviors directly following sex."