When, in 2002, Allison Pearson wrote I Don't Know How She Does It, she exposed, for the first time, the mayhem and exhaustion of a modern working mother. Yelps of recognition came from all over the globe. It was an international bestseller.

But in the short time between then and now, there seems to have been yet another seismic shift. Everybody is exhausted, not just working women with children. We're all run ragged by what social commentators refer to as 'the breakneck pace of life', or the 24/7 society that never sleeps.

What research points to is our inability to switch off and relax, either because of internal anxieties or those placed upon us by a boss, by society or by all of these things. The new technological age that was supposed to bring us freedom by allowing us greater flexibility is, in fact, slowly working to destroy us. It is as if we have made a pact with the devil. We'll work at home but we'll do so until 1.30am. We can leave the office at 7pm on a Friday - although we're too tired for a movie - but it means we'll be looking at and responding to emails on Sunday. Once at home, we are often too tired for guests or for dinner out (restaurants now mean seeing people texting and tapping BlackBerries, a nauseating sight when you are trying to have a rare work-free supper yourself). When we do climb the stairs to bed, our heads fuzzy with wine and crap Friday-night television, we have trouble sleeping. Sex is off the agenda, because, yes, we're too tired for that, too.

Only recently a friend of mine reported back to me - with a wry tone of recognition - a conversation she had found herself having on the prevailing mood of exhaustion-induced sexual disinterest: 'It's like this for me,' her friend had told her: 'Left boobie, right boobie. That's fine. Night night.' 'About three minutes usually does it.'

I felt embarrassment, mixed with the jolt of realisation that a barrier had come down; that exhaustion was now so integral to our lifestyle that it provided a cast-iron excuse for pretty much every social, physical and emotional failing. The same friend, not really existing among the ranks of the truly exhausted - and we'll come on to those - but nevertheless a consummate hostess and an excellent social barometer, added: 'Ten years ago I would have been horrified if a guest fell asleep at dinner. Now it happens all the time. And you just think to yourself, "I understand, everybody is so tired, working late, travelling, more pressures, worrying about things." Now when I ask people to supper, I'm never offended when they tell me they have a hell of a week coming up and could they please wait to see how they feel. Nor do I mind when one half of a couple comes on their own.'

Statistics bear this out. Only recently, a survey commissioned by Legal & General found that 42 per cent of the 5,000 people asked said that lack of sleep was their biggest health concern, followed by 34 per cent worrying about low-level, general fatigue. More than a quarter said they were stressed and another quarter admitted to depression. It was concluded that working long hours combined with not seeing enough of friends and family is about to threaten our health. These statistics confirmed those produced less than three weeks earlier by the Chartered Management Institute, whose 'Quality of Working Life' report showed that more than half of us experience feelings of constant tiredness at work and even more of us suffer from insomnia.

'Many of my patients experience a deep sense of shame about their feelings,' explains Dr Nick Read, author of Sick and Tired, and a consultant physician and psychoanalytical psychotherapist who works to help people cope with exhaustion and other illnesses that have no clear cause or pathology. 'They feel they are failing and fear that people might see that they might be failing. We are all absolutely terrified that if we don't just carry on we might be sacked. People know now that they don't have a job for life and that if they take time out to recover, somebody else will come along to fill their shoes.'

Read has a good point. When I began thinking about this topic, I went through my address book, contacting the most stretched people I knew. One friend admitted that, the night before, her husband had been on the computer working and answering emails until 2.30am and had got up at 5.30am with his mind racing. 'He's always exhausted at the moment,' she said.' And then she rang back. 'He can't really talk to you in case his clients think he can't cope. I'm sorry.' Another friend who had recently told me about how her sister was exhausted after a promotion, working flat-out all year without so much as one week of holiday, emailed me saying, 'I don't want people thinking she can't cope, and I really don't want her thinking I'm going round telling everybody she can't cope. There's no way she'll talk to you.'

What is this? The new taboo? The new infertility? Why is everybody talking about it in private but not in public? But, when you consider it properly, who can blame them? Who, in this current every-man-for-himself climate, wants their boss to think they can't cope, especially when there are mortgages andfamilies at stake? Who wants their colleagues to think they can't hack a bottle of wine after work because, frankly, they're just too old to cope with the consequences?

My friend Miranda (not her real name, to protect her job, of course) is a good example of this. She's Oxbridge-educated, sharp, smart and, with the help of a fantastic nanny, unconflicted about being a working mother. I have never heard her moan about the pull of home/work. She adores her job; she's a coper. Her company is in the middle of a major restructuring programme, creating many redundancies. When I asked her how she felt right now, she replied: 'Yes, I am pretty exhausted. I leave my desk only to go to the loo. Nine till 6.30, no lunch, every day, aargh. Then the tube, an hour each way. The really lovely thing is seeing my boys early in the morning or at 7.30 at night. But then my husband and I just drink and talk and sleep - then back to work again. It's stimulating work but one feels pulled, strained, stretched, no time to joke or chat. The main mode is irritability, impatience, barely suppressed rage, indignation. I don't have physical exhaustion, it's just that the ability to notice, relish, enjoy gradually leaches away. One feels oneself becoming poor company, all is inwardly focused. Going to an opera or play or even a film is anathema - these things need concentration which I cannot give. Books are fine, however, thank God! Not cookbooks, though - or rather I read them as escapism while eating my Waitrose ready-meal moussaka.'

But is it really worse now? What about the war? Surely people were exhausted then? People were losing their husbands, for heavens sake. Read makes a convincing case for it being a fundamentally different experience, tiring in its own way, of course, but not the same kind of pervasive, life-sapping exhaustion that seems so commonplace now: 'You talk to people about the war and many will say it was the best time of their lives,' he says. 'They were living and socialising and supporting each other. There was a tremendous sense of the problems and the dangers being overcome together. The threats were all external but they were facing it all together; there was a real sense of emotional togetherness, whereas nowadays the everyday threats to our lives are much less obvious. The emotional situations people find themselves in are more difficult to deal with.'

Will, a 34-year-old classics master at a major public school, concurs, blaming the way we live in a globalised society, linked by electronic media: 'Email robs you of all those moments when you are supposed to be in repose - glancing at the paper, having a quick fag with somebody. Everything translates to screen time now and that has a major debilitating effect. And like many other people, I have a network point at home. I'm checking my inbox all the time, when I should be resting, and I get this feeling of isolation from it.'

Will's wife adds: 'Will tries to be the perfect dad, the perfect husband, the perfect employee. Yesterday he worked all day, he came home for half an hour because he wanted to put our little girl to bed, then he worked all evening and got to bed at 2am. He's completely knackered.'

My own husband is no stranger to this pull. Last year, a month after he started a new job and we moved to a new house in an area where we knew nobody, we had our second child. Our first was only 18 months old. The second wouldn't sleep for more than two hours at a time. I became exhausted by this and by having a toddler who did not allow me to catch up during the day even though I had help. (I eventually got ill.) So far, so normal. My husband was determined to help. He got up in the middle of the night when I couldn't manage it, but it took its toll. He became exhausted by the dual pressures. When he got the chance to rest on holiday, his body crashed.

In retrospect, of course, it is ridiculous that my husband felt compelled to get up. But, as he explains: 'To stay asleep would have made me like all those old-school men I despise who don't seem interested in their children. I couldn't be that person to myself.' It is also ridiculous that I didn't sleep during the day when I could, but then I was worried about 'protecting' my toddler's feelings. So we were both to blame for the sorry state we got ourselves into, but both of us were responding to a change in society's notion of parenthood.

When I told a much older friend this story, a wise mother of two generations of children from two marriages, she was uncomprehending: 'The parent/child relationship has changed so fundamentally. Two generations ago, Peter and I would have gone out on walks together at the weekend, knowing that our children were happily playing on their own. As a child, I was always down the railway line, playing with other children, which is, after all, what children want to do. But now, because of what goes on, there is this terrible anxiety about leaving them on their own even for two seconds. We have to be with them all the time and that makes even a supposedly relaxing thing like a weekend walk - with the children dawdling at a different pace - stressful and exhausting.'

Read's thesis is that the increasing levels of exhaustion and depression have their roots in exactly these kinds of rapid changes to what we call society: 'When Mrs Thatcher famously said, "There is no society", she was perhaps responding to the notion that the population of the Western world has lost its social identity.'

There can be no doubt of the positive changes that have taken place in the last 50 years. Life in Western countries is much better than it was during the early part of the last century - we eat better, we are richer, we are healthier, there are more opportunities - but then, as Dr Read asks: 'Why, in the midst of so much excitement and opportunity, has life satisfaction declined so much? Why is depression the commonest illness in the Western world? And why, when most infectious diseases have been conquered and rates of heart attacks and strokes have been reduced, do so many people report that they are feeling ill?'

In his book The Third Wave, Alvin Toffler writes that human civilisation has gone through three major cataclysmic shifts - we're currently in the change from an industrial culture to a globalised one - and that each wave has been associated with some kind of ill health. The stress and exhaustion doctors are seeing in patients now, it is argued, are the same as those in middle-class England in the late 18th century. They just have different names.

Read argues that 'functional illnesses' such as constant tiredness, the inability to sleep, anxiety that makes you ill, are caused by the body's failure to adapt to social change. We travel to the other side of the world in a day, we communicate within seconds. Modern technology informs us, educates us, but it means that we are constantly threatened by global catastrophe, climate change and terrorist attack.

I'm not suggesting that I or anybody else goes around worrying about these things over and above the concerns of our families or careers, but it is in the background. Last week, I ran a small experiment. It was during the evening and my children were sleeping (my husband was working late, of course). I vowed not to check my email. At 8.15pm, I heard my phone bleep with a text. I went to the hall and picked it up and responded. It was about work. I walked past my study. I went in, abandoned my experiment and checked my inbox. I went into the sitting room, switched on the television and was faced with a documentary about the mother of a three-year-old girl who had been bundled into a car, driven to a hostel full of paedophiles and raped. I switched the television off. Was this modern life? The human compulsion is always to hark back to a better age, but I found those 15 minutes profoundly depressing.

Richard Hudson, a historian, points to the fact that, in its own way, modern life is exhausting because we often feel compelled to live by its standards, even though technically we have a choice to live otherwise. 'Back in the Sixties, it was predicted that Noughties living would be easier,' he continues. 'Everybody thought we would benefit from the invention of machines to carry out the more mundane tasks in our life, leaving us far more time for leisure and relaxation. And yet the opposite has happened. These domestic machines exist and yet we're more stressed, more pushed for time, more exhausted, because we have been liberated to do so much else.

'We are all, broadly speaking richer,' he says, 'which in itself discourages a sense of dependency on others. In the past, poor people knocked on each other's doors and asked for help. I bet you wouldn't do that, however exhausted you were.'

He's right. I wouldn't, even though I now live at least three hours' drive from my parents, which in itself is a small example of how families no longer provide help in the way they used to.

Often it takes the body packing up entirely to force a change. The link between exhaustion and ME is proven. Sufferers of ME usually point to exhaustion from pressures, such us moving house, moving jobs, having babies, which is then tipped over the edge by them getting a virus. Suddenly, they find they have ME. The discussion about what causes ME is so highly charged that Dr Simon Wessely, who runs the chronic-fatigue research unit at Kings College Hospital, has decided to stop talking about it in public: 'I am sickened by the politics of it,' he sighs. 'It raises vitriol and passion.' All he will say is that 'excessive activity and pressures are risk factors'. The message seems to be that feeling exhausted won't necessarily lead to ME, but it weakens the system making it more susceptible.

Joanna Hayes, a 39-year-old commercials director, had a narrow escape seven years ago. The story she recounts is revealing about how society tells us we can have it all, be whoever we want to be, without warning us not to embrace the dream too hard for our own good: 'I ended up having a breakdown,' she says. 'I found myself in a hotel room in Poland completely alone in every sense. I hadn't had a relationship for years. I had nobody to talk to, no proper friends. I was addicted to four grams of cocaine a week. I worked all day, every day, flying across the world on my own, pursuing this dream I had to be a director. And I was exhausted. Everything became about me as the director, not me as the person. I'd work all day, and party all night, literally all night because that's what the profession demanded. And that meant that I could never work without coke in my back pocket to keep me going.'

It is clear that Joanna was suffering from an affliction identified by the philosopher Alain de Botton in his book Status Anxiety, in which he suggests that adult life can be defined by two great love stories, the first being the story of our quest for sexual love, and the second, our quest for love from the world. De Botton asserts that our melancholy and dissatisfaction in the midst is due to the anxiety we have over our own status and the envy of those to whom we consider ourselves equal.

Sam Leith, a 32-year-old rising star at The Daily Telegraph, where he works as literary editor, critic and columnist, feels the same way. With no wife, no girlfriend, no children, he admits his life is his work. He works until 1am many nights and often all weekend. 'If the essence of self esteem is asking yourself, "What is the point of me? Who am I?" then in the absence of those other things in my life, it has to be what I do. I am helped by an awful lot of alcohol to get through my life. I'm a great self-medicator. I keep going on this hamster wheel because if I stopped I might think, "What is the point?" Alcohol is the only way I can compress relaxation. There I am for hours reading and writing and writing and then I need this huge drink.'

How is your physical health? I ask him. 'Horrendous. I am exhausted much of the time. I drink too much, smoke too much, don't eat. I fall ill. I watch crap TV, play internet poker, but you see all of this is self-inflicted. It's all to do with staying on the hamster wheel. Sooner or later I'll have to slow down. Either I will get a wife and 2.4 kids, or I'll end up a gibbering wreck in the Priory or I'll arrive at a place where I'm happy with myself. At the moment, I'm writing as much as I can.' He pauses. 'This all sounds very bleak. I'm quite a cheerful person really.'

Professor Ian Gilmore, chairman of the Royal College of Physicians and a liver specialist, says sternly, 'People who drink regularly in order to overcome feelings of stress, tension and exhaustion are going to run into dependency problems. If you drink a lot and then something very stressful happens, the drinking escalates. Many people who are exhausted use it as a sedative, but then find they wake up after a few hours unable to get back to sleep. Alcohol suppresses rapid eye movement and disrupts the normal sleep pattern.'

Exhaustion can often mask another kind of status anxiety: greed, or a kind of acquisitiveness propelled by envy. De Botton talks about it in his book and Read sees it in his patients. People who work round the clock for the big house, the big car, the 'big' life but have very few core values. This is confirmed for me by Kate Murphy, a 42-year-old fund manager who now works part-time. As a woman used to the cut-throat culture of the City, she resents the 'poor-old-me' culture where people bemoan their hours and their lifestyle but fail to see that it is their own greed keeping them there: 'What you are seeing is well-educated, intelligent people losing the ability to make choices about their lives,' she says. 'They get exhausted and they think they have no choice, they think they cannot stop, but if you really examine the question, you'll find that they have a socking mortgage on a really big house, they educate three children privately, they go on foreign holidays. It's all about status. They've lost touch with the ability to analyse their lives. Why do people moan, "Oh God, I'm going to Morocco, I'm exhausted, I can't face the journey"? Why don't they just stay at home in the house they've no doubt spent a fortune doing up and read a book in the garden? We have seen so much of the good times, of benefiting from the success of capitalism. I think exhaustion masks fear: fear of not keeping up, fear of war, fear of terrorism which horrifies us but which makes us hooked to it through news reports. People have to realise that they do have a choice to change some parts of their life, at least.'

Clearly not everybody who is exhausted is pursuing the capitalist dream, but perhaps most people have lost a sense of control, of seeing how even small changes might make a difference.

Joanna Hayes, sobered by her breakdown, still drinks heavily for pleasure and still works phenomenal hours, but with a few fundamental differences: 'When I work away, I never stay in a hotel without a spa. I never drink when I work, I go to bed early and eat healthily. I spend money on treatments instead of booze and drugs. It's the only way my body can cope.' And after a gruelling few weeks, she will not immediately visit her boyfriend who lives in another city. 'He says, "How come you can work flat-out for three weeks and, when you're with me, you haven't even got the energy to use the remote control?" I've realised that, as much as I want to be with him when I feel like that, I know that these are the times when it's kinder to myself to be on my own.'

I've come to view exhaustion rather like Micawber thinks of his profit margins. The smallest amount makes the biggest of difference: 'Annual income 20 pounds, annual expenditure nineteen nineteen six, result happiness. Annual income 20 pounds, annual expenditure 20 pounds ought and six, result misery.'

'I guess it's all about knowing your limits,' a particularly frazzled, twitching friend told me. It was the wisest thing I'd heard for weeks. I'm going to start to try to find mine.