Arguing all the time? Spark missing from your marriage? It could be that one of these outside influences is to blame – and some of them may come as a surprise...
1. Sleepless nights
Suffer a bad night's sleep and you're far more likely to argue with your partner the next day, according to researchers at UC Berkeley. Indeed, just one sleepless night can be enough to tip couples over the edge. The solution, of course, is to take steps to improve your sleep: go to bed at the same time every night, keep digital screens out of the bedroom, cut down on stimulants such as caffeine and try to relax before bedtime.
Read our tips for better sleep
Ah, so that's why you haven't been sleeping! More than one in five men and women say snoring puts them off being intimate, according to a recent OnePoll survey of 500 UK adults. And nearly half of the women surveyed claim snoring has a negative effect on their relationship.
Ready to tackle snoring? Lifestyle changes such as losing weight, cutting down on alcohol in the evenings and giving up smoking can all help. Or various anti-snoring treatments, such as nasal strips and oral devices, are available from pharmacies.
Find out how to beat snoring
3. Watching TV
Spend most of your spare time glued to the TV instead of paying attention to your partner, and your relationship will suffer. But there's another reason why your small-screen favourites may be having a negative effect on your lovelife: they can give you unrealistic expectations of romance, says research from Albion College in the US.
'I found that people who believe the unrealistic portrayals on TV are actually less committed to their spouses,' says study leader Professor Jeremy Osborn. 'My hope would be that people would take a look at their own relationships and the relationships of those around them. How realistic are your expectations for your partner and where did those expectations come from?'
Yes, it's a great way to keep in touch with far-flung friends and relatives – but are you spending too much time on Facebook at the expense of your relationship? Heavy social network use is a 'positive, significant predictor of divorce rate and spousal troubles', says a study published in Computers in Human Behaviour.
The reason? Aside from trespassing on that all-important 'you-and-me-time', seeing other people's updates about their attentive partners and romantic holidays can make our own relationships appear inadequate. So spend less time on Facebook – and more face-to-face time with your partner.
Find out why it isn't a good idea to Google you ex
5. Your pet
...particularly if Rover or Tiddles is a needy type. If you're in a new relationship, a certain amount of adjustment is required to ensure you all get along. A few pointers from Battersea Dogs & Cats Home? Introduce changes gradually; don't be tempted to lock your pet away; try to distract them with toys or food. And if you're in a long-term relationship and your pet sleeps on your bed, ask yourself whether this could be having an adverse effect. Is it disturbing your sleep, for example? Or stopping you having sex? It could be time to invest in a separate bed for your pet!
6. Your commute
If at least one of you has to travel a long way to work... beware! Risk of separation is 40 per cent higher among long-distance commuters than other couples, says a study from Sweden's Umeå University.
A commute of just 45 minutes each way can be enough to make a difference – due to factors such as increased stress, more time apart and an imbalance caused by the non-commuting partner having to shoulder the bulk of the family and household duties. So always consider the impact of the commute before taking a new job, and make a joint effort to get the balance right.
Read these five secret driving tips from a chauffeur
7. Losing weight
Sticking to a new diet and exercise regime? Good for you! But when one partner loses a substantial amount of weight, it may put a serious strain on the relationship, according to US research. The reason? The non-dieter may feel threatened or insecure, and even try to derail their partner's efforts. The solution, of course, is to agree to make healthy changes together.
Visit our weight loss section for tips on losing weight in a healthy way
A third of UK couples admit that football-related arguments are responsible for the most tension with their partners, according to a 2014 poll commissioned by HTC. But let's not lay all the blame at football's feet: if one partner is an obsessive fan of any sport and the other would rather watch paint dry, it can cause problems.
The answer lies in compromise and keeping the relationship equal. While it's important that you're both able to indulge your separate interests, ensure you always make time and space for one another, too.