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“Birds of a feather flock together.” Research in psychology has repeatedly shown that when it comes to interpersonal , this old adage is true far more often than not. Especially for long-term intimate relationships, we tend to prefer partners who come from the same ethnic or racial group, religious tradition, socioeconomic status, and level of . And in married couples, we see far greater similarity in than would be expected by chance.

Selecting a mate who’s similar to yourself is known as assortative mating. Psychologists have found all kinds of traits that are correlated between partners, yielding plenty of evidence for assortative mating. This goes both for physical traits, such as the distance between the eyes, as well as psychological traits like personality. What’s more, assortative mating isn’t a uniquely human phenomenon: Throughout the animal kingdom, individuals seek out mates similar to themselves, whether it be in terms of size, strength, or rank in the pecking order.

One personality variable that has not been explored in depth within the context of intimate relationships is chronotype. This is an individual preference for a particular time of day. On the one hand, morning “larks” prefer getting up and going to bed early, and are at their early in the day. On the other hand, night “owls” like in and staying up late, and don’t perform well until afternoon or evening.

Like other personality traits, chronotype extends along a continuum, with a few extremes at each end, and most people clustering in the middle. If you don’t have a preference for morning, afternoon, or evening, then you’re a neutral chronotype. But if you’re a morning lark or a night owl, you definitely know it — and you’ve probably got a pretty good idea of your partner’s chronotype as well.

To some extent, chronotype can be shaped by circumstances. If you’re a baker, you’ve got to be a morning lark, since your goods have to be ready to sell by other people’s breakfast time. And if you’re a jazz saxophonist, you’d better be a night owl, because that’s when all the gigs are.

But there’s also a biological component to chronotype. For one thing, chronotype changes with age. Young children tend to be morning larks, but shift to become night owls after , only to trend back toward a morning preference by later adulthood. There’s even a slight difference, with females tending more toward morning-type and males toward evening-type.

Even the timing of bodily processes is different in morning-types and evening-types. For instance, body temperature cycles through a daily rhythm, but it peaks an hour earlier for larks than for owls. (Yes, researchers really do call morning-types “larks” and evening-types “owls.”)

Overall, health outcomes are better for larks than owls. Morning-types experience fewer health issues, less incidence of , and report higher levels of subjective well-being. So apparently there’s some truth to the saying, “Early to bed and early to rise, makes a man healthy, wealthy, and wise.”

Chronotype even predicts a person’s attitudes toward . Both male and female evening-types are more likely to approve of — and engage in — short-term sexual relationships, whereas morning-types tend to prefer long-term relationships. This makes intuitive sense: If you want to find casual sex partners, you’ve got to be around late at night, because that’s when all the short-term maters are up. It’s much harder to find a hook-up at a Starbuck’s at six in the morning.

Prior research has shown that couples tend to select partners who are similar to them in chronotype — larks tend to marry larks, and owls tend to wed owls. But of course, most people are in the middle range, where small differences in time preferences could possibly lead to conflict. In particular, differences in preferred time for engaging in sex could lead to dissatisfaction in lark-owl couples. This is the issue Polish psychologist Paulina Jocz and her colleagues at the University of Warsaw investigated in a recently published study.

Specifically, Jocz and colleagues asked three questions:

Does a person’s chronotype extend to a preferred time for having sex?

Does a difference in chronotype between partners lead to a decrease in sexual and relationship satisfaction?

Are morning-types happier in their long-term relationships compared with evening-types?

To answer these questions, the researchers recruited 91 heterosexual couples who’d been sexually active in their current relationship for at least six months. Each participant individually responded to a set of questions without being able to compare answers with their partner. The questionnaire included items measuring:

Morning or evening preference for various daily activities.

Sexual satisfaction.

Relationship satisfaction.

Preferred time of day for sex.

Actual time of day when they usually had sex.

The results paint an interesting picture of the intimate lives of other people. First, and to no surprise, sexual satisfaction and relationship satisfaction were strongly correlated for both males and females. Further, both partners generally agreed on their level of sexual and relationship satisfaction. These findings concur with other research showing that the quality and quantity of sex are good barometers of the emotional state of a relationship: Happy couples just have more and better sex than unhappy couples.

In contrast to previous research, no evidence for assortative mating in terms of preferred time for sex was found. Rather, differences tended to split along gender lines: Male larks preferred having sex in the morning, while male owls preferred doing it in the evening. But women, whether morning-type or evening-type, preferred having sex right before going to sleep.

Jocz and colleagues acknowledge that these two most preferred times for sex — early in the morning and late in the evening — could also be shaped by people’s daily work schedules. Most of us work during the day, so mornings and evenings are the only times we usually spend with our partners. Interestingly, there was also a small peak in sexual interest in mid-afternoon, but only for evening-type males. Make of that what you will.

And what about the morning men? Even though they wanted to have sex in the morning, their partners almost always preferred sex at night. As a result, the timing of sexual activity was dictated by the female’s preference, not the male's. Although this mismatch had an impact on relationship satisfaction, it didn’t affect the frequency of sex — apparently, “lark” husbands accommodate their “owl” partners’ desires. In other words, they accept that sex on her schedule is better than no sex.

The third hypothesis proposed that morning-types would be more satisfied with their relationships, because in general, they report higher levels of subjective well-being than evening types do. Again, the data split along gender lines: Morning-type males did not report greater relationship satisfaction than evening-type males, as they had been expected to. The researchers speculated that morning-type males were likely frustrated because they weren’t getting sex when they wanted it. However, morning-type females did say that they were happier with their relationships than evening females, in line with the hypothesis.

Differences in a couple’s chronotypes can lead to sexual and relationship problems. The researchers suggest that counselors take into account morning and evening preferences when working with troubled couples. But it would be a good idea for all couples to carefully consider their own and their partner’s chronotypes.

If you’re a “lark” husband, try to negotiate at least an occasional morning romp. Always giving in to your partner’s preferences could lead to resentment, affecting the relationship satisfaction of both spouses. And, women, if your husband is a “lark,” try to let him have it when he wants some of the time; for example on the weekend, when you don’t have to go work. You’ll both be happier for it.