There are dozens of books on how to solve individual sleep problems, but I know of none that focuses on the couple. Yet millions of adults sleep with another adult and for them sleep is a complicated, changing and often challenging experience.

What happens when one partner snores? Steals the sheets? Prefers to sleep in the nude? To address these and other questions, I asked more than 40 couples to describe the struggles and achievements of their bed-sharing. My 88 interviewees ranged in age from 21 to 77 and had been sleeping with their partners for between six months and 51 years. I tried to respect what I thought might be the deepest secrets by not asking much about sexuality, but some people did talk about such matters. I’m sure that secrets were withheld from me, but much was revealed.

Sleeping takes up between a quarter and a third of our lives, and many of my interviewees spoke of how important sleeping together was for them. It was a time for intimacy and pleasure, and some said that they would feel quite alone without it.

Bed-sharing is an achievement of co-ordination: where to put one’s head, body, arms and legs; where to put one’s pillow; when to talk and not talk; when and how to touch each other; what to do if one partner wakes during the night.

In their first nights together, couples often sleep entwined in ways that give one or both of them sore shoulders or arms. Nobody whom I interviewed could tolerate such discomfort night after night.

Donna [25, married for two years]: “When you’re first married you have this notion that you must sleep intertwined, but we learnt that it was OK not to do that — we could spread apart. If he had his arm under my head, I’d get a kink in my neck.”

Couples must learn to sleep together. A person may be able to say from the start what he or she wants or needs in order to sleep comfortably, but often they don’t know until their partner does something that makes them uncomfortable, annoyed or frustrated.

Sometimes achieving tolerance meant thinking differently. If, for example, one partner at first blamed a character defect in the other for a sleeping problem (“If he wasn’t so selfish he wouldn’t snore so much”), they might eventually come to view the snoring as something that couldn’t be helped.

All the couples had developed routines that covered how they got to bed, what they did once both were there, how they fell asleep, how they slept and how they woke. The routine of getting to bed might include watching the evening news together, after which one partner would check that the outside doors were locked while the other started the dishwasher. One might check on pets and children and turn out lights. Then both would go to the bathroom, brush their teeth, undress and put on their night attire, if any. They almost always climbed into the same side of the bed in the same way. One would set the alarm clock. One would turn out the lights. They would snuggle, talk, then fall asleep.

Even when both were asleep they would continue their routine — for example, both facing away from the middle of the bed and staying apart, or rolling over in synchrony.

Routine makes it unnecessary to think much, decide much or negotiate. It helps most couples to sleep easily and well.

The bed

Not surprisingly, couples often differ in their idea of the perfect bed. Almost 80 per cent of those I interviewed shared a queen-size or king-size bed. Some could not sleep if they were touching their partner, and typically they did not want their partner crossing to their half of the bed during the night. Some were good at defending their territory and moving an invading partner away. Steve [30, married for five years]: “She tells me that I’m hogging too much of the bed, so she keeps pushing me over to the point where eventually I’ll get up, walk round the bed and go to sleep on the other side where there’s more space.”