It's a truth universally acknowledged that the act of physically sleeping together does not always equal relationship harmony. What with snoring and 3am tussles over who gets the duvet, there are plenty of pitfalls to disrupt a good night's sleep with your other half.
However there is one easy step you can take to bring happiness - not just in terms of sharing a bed but to your entire relationship; and that's sleeping naked together.
A new survey from Cotton USA has found couples who sleep in the nude are happier than their clothed and PJ'd counterparts.
The cotton experts quizzed 1,004 Britons about their sleeping habits and whether or not they felt happy in their relationships.
They found 57 percent of couples who slept in the nude claimed to be happy together, compared with 48 percent of those in pyjamas, 43 percent of nightie wearers and 38 percent of those in onesies.
Suffice to say, it's not a good day for that panda-themed onesie lurking at the bottom of your wardrobe...
The results are not exactly rigorous; there's nothing to indicate why nudity affects happiness and it could easily be the case that the happier the couple, the more likely they are to sleep naked together (as opposed to nude sleeping actually causing that happiness).
But Stephanie Thiers-Ratcliffe, of Cotton USA, suggested such nudity encourages openness and intimacy, and therefore leads to greater happiness between partners.
"There are many factors which can affect the success of a relationship," she said. "But one factor which is often overlooked is the bedroom environment."
With that in mind, here are a few other unlikely factors that have been proven to affect happiness in relationships:
Five unexpected factors that bring happiness to a relationship
1. Sleeping less than an inch apart
A survey of 1,000 people conducted as part of the Edinburgh International Science Festival earlier this year found that partners who slept less than an inch apart were more likely to be content with their relationship than those maintaining a gap wider than 30 inches.
And those people who touched each other during sleep were happier than couples who had no contact.
Professor Richard Wiseman, a psychologist of the University of Hertfordshire who led the study, said: "One of the most important differences involved touching. Ninety four per cent of couples who spent the night in contact with one another were happy with their relationship, compared to just 68% of those that didn't touch."
Prof Wiseman, the author of Night School, which examines the science of sleep and dreaming, said: "The key issue is if you have a couple who used to sleep close together but are now drifting further apart in bed, then that could symptomatic of them growing apart when they are awake."
Arguing can be beneficial to a relationship, a 2012 study found
2. Having no children
A controversial statement, this one, but one of the biggest studies ever of relationships in Britain found that couples without children had happier marriages. More than 5,000 people of all ages, statuses and sexual orientations were interviewed in detail over a two-year period about relationships and what made them happy, in this year's "Enduring Love?" project by Open University.
The survey's researchers concluded that childless men and women were more satisfied with their relationships and more likely to feel valued by their partner. However, they also found that women without children were the least happy with life overall, whereas mothers were happier than any other group, even if their relationships had suffered along the way.
The question of children also influenced intimacy levels among couples. Fathers were twice as likely to cite a lack of sexual intimacy as the biggest downfall of their relationships, while mothers reported that they wanted to have sex less often than their partners did.
According to a 2007 study from researchers at Rutgers University, New Jersey, feminism - in the form of men or women - brings stability and sexual satisfaction to relationships. In a survey of college students and older adults, the team found that women whose male partner was a feminist reported better relationship quality, while men with feminist partners experienced more sexual satisfaction and relationship stability.
Feminist women were also more likely than others to be in a romantic relationship, the survey found.
The results pretty much dismantle the common stereotype that romance and feminism are uneasy bedfellows, and also stamps on the myth that feminist women are unappealing to men.
"Contrary to popular beliefs, feminism does not disrupt men's pleasure in the bedroom," said Laurie Rudman, leading the study.
"If you're a woman paired with a male feminist, you have a healthier relationship across the board - better in terms of relationship quality, equality, stability and sexual satisfaction. And men paired with female feminists have greater sexual satisfaction and greater relationship stability."
Boston sex therapist Gina Ogden puts this finding down to the need for a natural ebb and flow within a relationship.
"If a relationship is based on authoritarian control, keeping one person on top and the other underneath, it gets old pretty fast - for both partners, really," she said. "In an egalitarian relationship, there is more flow of give and take. And that's the romantic tension. That tension - the sexual desire - is in that space between you where you're able to flow back and forth."
Small gestures have a big impact on the happiness of a relationship, according to an Open University survey
4. Saying thank you
It may not seem like the kind of thing to set your world on fire, but the same survey by the Open University quoted above also that found simple expressions of gratitude played a big role in maintaining healthy relationships.
Small gestures, such as telling a partner "thank you" and giving compliments were shown to be among the most important factors in preserving bonds between partners across all ages, statuses and sexual orientations. So you can forget about booking that five-star weekend getaway to Provence - it's the little things that count.
"What this study shows us is that couples need to keep investing in their relationships. It's reassuring to know, especially in these tough economic times, that it's the small gestures of appreciation and affection, rather than the big romantic displays that really make the difference," said Ruth Sutherland, chief executive of the relationship support organization Relate, which contributed to the study.
5. Angry, honest (but temporary) disagreements
Are screaming matches really the key to eternal happiness? Perhaps so.
A 2012 study from Florida State University concluded that marriages are happier and last longer when the couple in question fights. Researchers at the university found that fighting can actually be beneficial as long as it's temporary - because getting angry can help signal that certain behaviour from your partner is unacceptable.
Researcher James McNulty emphasized that it is normal and healthy to have temporary disagreements in relationships.
"People may experience long-term benefits by temporarily withholding forgiveness and expressing anger," he said.
It's worth noting that these "angry but honest" conversations made couples happier in general, but only to an extent. Out of the 100,000 people surveyed, 90 percent of the happiest individuals have never sworn at their partners.
So argue, but hold back on the insults is what we're taking away from that.
Words: Anna Brech, Photos: Rex Features