Charla Muller was reading Galatians 5.22-23 in her Bible study group when she decided what she was going to get her husband, Brad, for his 40th birthday. Perhaps disappointingly for him, it wasn't an iTunes voucher. Instead, she was going to give him the gift of sex for 365 nights.

As you know, Galatians 5.22-23 reads: "But the fruit of the spirit is love, joy, peace, patience, kindness, goodness, faithfulness, gentleness and self-control." What's that got to do with a scheduled sex marathon? "From that, I realised I needed to bring something to bear on our relationship to make it fruitful. We had been married for eight years and I wanted to reconnect with Brad, and give him a gift he would never forget."

Maybe, particularly in harsh economic times, modern couples who are enduring the dark night of the soul that is the long-term relationship should consider giving each other the same gift Charla gave Brad. Forget the downsides - the missed Everton-Man U penalty shoot-outs, the soreness, not to mention having to think of new ways to keep the bleeding thing interesting each and every night. Accentuate the positive. Think of the free nightly endorphin rushes. Not to mention how much you'll save by turning out the lights and giving up your subscription to Sky+. Think how close you'll be to your partner (even if, all things considered, you'd rather be playing online Scrabble, lying about your life on Twitter or taking your Second Life avatar to a roller disco).

What was Brad's reaction to this gift, I ask Muller. "He turned me down! He thought that scheduling time for intimacy would detract from its loveliness. He also wondered if he was up to it. He said, 'What would happen if I have a headache?'" So she drew up a list of ground rules, among which was that either party could decline on any occasion.

"Eventually he said, 'Let's give it a go.'" As Brad's birthday neared, though, the couple worried about logistics - how would they find the energy and free time (she works in marketing, he is a salesman), how would they ensure that the kids (aged seven and five) didn't intrude, and what if there was something really good on telly? "We agreed that TV couldn't trump intimacy, and that once we scheduled some saucy time, BlackBerrys and emails would be ignored." What about the kids? "They're old enough that we don't have to look out for them every five minutes, so we would often put them in front of a video. We were also much stricter with bedtimes than we had been before. We also weren't afraid to lock our bedroom door. Sometimes we had a great babysitter for our date nights."

What would have happened if Brad had offered the same gift for your birthday? Would you have said something along the lines of "Gee, Brad, hope you kept the receipt"? Muller giggles down the phone from Charlotte, North Carolina. "I'm not sure I would have accepted. In fact, when I turned 40, which was recently, Brad said: 'Well how about it?', meaning let's have sex for another 365 nights. But I didn't accept."

Wasn't Brad's initial reaction right - 365 days of scheduled sex is surely a turn off? What about spontaneity? "I felt the opposite. I felt the pressure came off. He no longer thought 'Tonight is a big deal, the only night we'll have sex this month is now, it's got to be really special.' And for me, before nightly sex, I used to guiltily wonder when I was going to have the time or desire. With sex every night it meant that I had to find the time, and that when it happened it was no longer necessarily a big deal." What about the desire? "The idea was that it would come." In fact, Muller writes in her book, 365 Nights: A Memoir of Intimacy, "Regular sex was allowing for feelings of health and wellness that sparked a desire to have more sex. Sex is a great stress-reliever too. A nice relaxing romp with Brad was a wonderful distraction from feeling like the world would crumble if I wasn't out there battling dragons 24/7. I could relax, feel those endorphins pinging around my body and forget about my bad day. And perhaps best of all, our intimate moments were making me feel younger."

It wasn't always that good. For instance, in her book Muller recalls the moment Brad said to his wife during what she calls, significantly, "the final stretch", "Could you stop grimacing? Could you at least pretend you're enjoying it?" And she replied, "How about you close your eyes?" He sighed (the brute!) and did just that.

But wasn't it an ordeal? "Let me tell you a story," says Muller. "One night we were at an Italian restaurant, feeling filled with wine, pasta and bread. A friend said to me, 'I really feel sorry for you - you've got to go home and have sex. I can go home and watch Saturday Night Live in bed.' I told her that we'd already had a quickie - we booked the babysitter an hour earlier. It really started the evening with a bang. So no, it wasn't an ordeal - at least, not most of the time." That said, Muller does write in her memoir: "Intimacy every day is trying. It requires stamina, patience, personal grooming and a work ethic I didn't know I possessed."

She concedes that before the birthday present, her and Brad's sex life had become pretty dreadful. "Brad was always wondering if he was going to have any sex this month, and I was guiltily wondering when I was going to have the time or the desire," she says. What about after his 40th? "He was newly energised. In the first month, when I asked him if he had any plans for the weekend, he listed all the things he would like to do, including taking me out for dinner. I was speechless. Before, he would have happily gone along with whatever I planned. Now he was looking at our diaries, initiating activities. It was as if we were dating again.

"The unintended beauty of my gift was its unconditionality. There was no need for Brad to wine and dine me at overpriced restaurants. Our house ran better because we were more agreeable, more helpful, more solicitous to each other, and our time together was truly about us, not the promise of special date-night sex."

This is hardly the first time that a woman (and it usually is a woman) has devised a project to revivify a long-term couple's sex life, and then written a book about it. The delightfully surnamed Esther Perel wrote a book called Mating in Captivity: Reconciling the Erotic and the Domestic; the less delightfully surnamed David Schnarch wrote Passionate Marriage: Keeping Love and Intimacy Alive in Committed Relationships. In The Surrendered Wife, Laura Doyle argued that women should stop telling men what to do and how to do it. "When I surrendered control, something magical happened," wrote Doyle. "The union I had always dreamed of appeared. The man who had wooed me was back. The underlying principle is simple: the control women wield at work and with children must be left at the front door of any marriage to revitalise intimacy."

Muller seems to be the opposite of Doyle's surrendered wife: she was not surrendering, but taking the initiative by offering daily trysts. "It's very passive aggressive," says Rowan Pelling, former editor of the Erotic Review. "It's like, 'Here's a lovely gift, but you have to do this every night on the dot - otherwise what kind of man are you?' I hate everything about it."

"There's been a series of these kinds of books from the US," says sex educator Petra Boynton of University College London. "They are very prescriptive and they may well end up making people feel guilty and inadequate.

They tap into a favourite theme of the media which is that more sex is better sex and what you need to do is engineer the means to have more sex, which means improving technique and buying accessories." Is this a heterosexual thing? "There are gay versions of the same thing - books about how perfect sex techniques will make everything all right. It won't. It's great to give the perfect blow job, if that's what turns you on, but not if it's to stop your partner having an affair. There's not enough in these books about friendship or being nice to each other - which sounds corny, but long-term relationships are not all to do with what you do in the bedroom."

Was sex a chore for Charla and Brad? "I won't lie to you," Muller replies. "In the last months we were going through the motions, but there was still a value in it, I think. Even when they were quickies and it wasn't a blow-your-socks-off honeymoon experience, it was still a good way of connecting. It made me look him in the eyes and him look me in the eyes and connect physically and emotionally." Did every night involve penetrative sex? "That's what worked for us. I'm not saying this is an idea for everyone. I would never prescribe what we did for people in a difficult marriage. My husband wouldn't leave me because we didn't have sex, but for deeper reasons."

"I think it's rather sweet," says clinical psychologist Linda Blair. "We create time for TV and domestic work, so we should create time for sex just as she suggests. I get a lot of my clients to plan dates like that and the anticipation and the getting ready are parts of the pleasure. The idea that, in our modern lives, we're all too stressed to have regular sex is wrong - we just need to plan more. So I like what Charla Muller is saying - except that she didn't really follow through on the idea that she could decline sex if she didn't feel like it. "

Blair argues that regular planned sex and other kinds of intimacy may have spin-off benefits for the children. "When parents have children, often the woman effectively gets married to the children and that's not a good role model. If the parents look as though their relationship is all about self-sacrifice, why would the kids want to have children themselves? Growing up looks like no fun to them. So we have lots of kids who don't grow up."

Blair adds that the couples who stay together after the children leave are generally those who have good sex lives while the children are at home. But she has a caveat: "I'm not sure about sex every day - that would tire even the fittest person."

And so it proved for Charla and Brad. There were several occasions (Muller mentions two) when Brad was not up to their nightly tryst. On day 305, Charla came to bed as game as ever with teeth brushed, face freshly scrubbed and pulling her hair into a pony tail, only to find Brad behind a copy of Newsweek. "You know, sweetie," he said, "I think I am going to pass tonight if you don't mind. I'm tired, I have a big day tomorrow and we've been having a lot of sex lately." In her book, she writes: "As if I hadn't noticed. I would have 'passed' about 200 times by now if the offer had been the other way round." Did you feel rejected? "A little, but mostly relieved. I wanted to snuggle down and go to sleep. I also liked the fact that Brad was able to admit that he didn't need to go that night. It was like a bridge had been built."

Muller declines to discuss details of what went on in their bedroom during that year, which is a shame because I had a lot of questions about cystitis, lubricants and the size of their bed (which in one photograph looks a little small for what she calls, disarmingly, "saucy time"). Her memoir, 365 Nights, is hardly as racy as the title suggests. "Some people think I'm this crazy, tawdry woman, but they clearly haven't read the book. The book is about intimacy, about how to reconcile sex with one's faith, one's children, one's role as a wife and mother."

Pelling is sceptical: "It takes a lot more to make a relationship work and that requires sensitivity and creativity on both sides." What would Pelling recommend to make a long-term relationship flourish? "Holding hands and saying 'I love you' now and again is a good start. I don't think sex every night is a good idea. Frequency doesn't matter - making an effort to make your partner feel special does. Foreplay helps, as does lingerie." Why lingerie? "To show you're making an effort."

Boynton worries that such books as Muller's might be used, futilely, by people to stop their partners having affairs. "Offering a gift of sex like this is almost a recipe for resentment in a relationship. Charla Muller writes almost as though she's superwoman - she talks about doing four loads of washing before she takes the kids to the nursery! And then she's supposed to become this geisha woman servicing her husband.

"There's sometimes a sense, especially from religious groups, that it's your duty as a woman to provide sex, and that even if you don't feel like it, you should just do it anyway. That attitude encourages women to chastise themselves. That's no good for a relationship."

Boynton thinks readers of 365 Nights - particularly women - should be sceptical of Muller's birthday present : "This is one person's experience. There are no mandatory guides you have to follow to the letter," she says. "If it doesn't appeal to you, don't think you're abnormal. You may want to have sex 365 nights a year - but with your neighbour. That's OK. Your desires are important."

I put these remarks to Muller. "I don't disagree - all I would say is that this worked for us. It's different for every couple. My husband knew I loved him, but that wasn't enough. Everything got better because of what we did. And I don't think there is anything wrong with a wife trying to please her husband."

What happened when Brad turned 41? "I was giddy with the notion that I didn't have to have sex," Muller writes. "I was bursting with deep satisfaction that I had carried it through." That makes it sound like a task, I suggest. "It wasn't all task, believe me," says Muller. "Nowadays we're much more relaxed with each other and the house isn't full of unspoken tension. " If nightly sex had such great benefits on your relationship, why did you stop? "It was partly a challenge to see if we could do that every day for a year. And we proved we could. Now we're reaping the benefits. Before, sex was abysmal. Now I have discovered I do have time for quality sex on a regular basis, which wasn't what was happening before. So now intimacy - and that includes sex - is better than it's ever been."

Muller concludes with some advice for married couples: "However often you are doing it, double it. And six months from now, double it again. It's proof that you're here, alive and very together".

• 365 Nights: A Memoir of Intimacy by Charla Muller is published by John Blake, £11.99.