On a Sunday morning in early June, Kate, a 36-year-old counsellor, was sitting on a sofa, drinking a cup of tea, and saying she didn't think she could go on any more. "I can't see a way out," she said. "I look at my life and I don't see any possibility of hope." She dipped her head and put a thumb up to her eye to brush away a tear. "I know I've said it before. But this time... I've come to the end."

Kate said: "I don't know what to do. God, I could just give up. Yesterday I had this feeling that I could just give up my responsibilities. I could become derelict and hopeless. But that's not the way to go, is it? I have a child. I have my job. Something's got to give. I don't know what, but something's got to give, because I'm at breaking point."

I was talking to Kate about exhaustion. I should say, first of all, that Kate is not her real name - she does not want me to use her real name. What if her boss knew the state she was in? For one thing, she is responsible for the wellbeing of other people - people who are supposed to be more vulnerable than her. Although sometimes, these days, she's not so sure.

What state is Kate in, exactly? She is drained beyond belief. Her facial expression reminds you of one of those young combat veterans you see in war photography; she has a "thousand-yard stare". Her facial muscles are somehow bunched up. Her body, she says, aches all over. She is often dizzy and nauseous. She describes her mental state as "foggy" and "fuzzy". On top of this, she has persistent bacterial and viral infections - this month she has had a cough; last month she had aches and fevers. She has just finished two courses of antibiotics; her cough, she says, is dying down. But when one thing dies down, another always springs up to take its place.

Once or twice a day, while she's working, Kate feels as if she's going to faint. It's as if her entire system is shutting down. "Something descends," she says. "I feel draped in it. It's like a curtain coming down." What's the explanation? Kate does very little physical labour. She does not run, or cycle, or walk long distances, or carry heavy loads. Her exhaustion may feel physical, but it is coming from somewhere outside the physical realm. "It's weird," says Kate.

But there's nothing weird or abnormal about Kate. She is one of an enormous number of people with a similar constellation of symptoms - millions of people at the end of their physical, and spiritual, tether. Frank Lipman, a South African doctor working in New York, has identified the condition in hundreds of his patients - he has a word for it: "spent". Lipman says that feeling spent is an understandable response to the 21st century. If you put a human being in a modern city, and add computers, mobile phones, credit cards, neon lights and 24-hour shopping, he says, what do you expect?

An important thing about Kate's situation is this: every night she spends seven hours in bed. It's just that she does not spend all of this time asleep. If she goes to bed at 11.30, it's sometimes an hour before she loses consciousness. Then she wakes up in the middle of the night for another hour or two - or, worse, at 5am, which means lying awake, increasingly desperate, and possibly falling into a doze just before her alarm rings at seven.

"Then I press the snooze button," she says. "Then the alarm rings again - that noise! Oh! And I press the snooze button again. I do this maybe four times. And I just... can't... move. I feel as if I'm glued to the bed. I feel paralysed. And really, really desperate. It's another day - another day of being late for everything, which just compounds the problem, another day of feeling worse than I did the day before. Because it is getting worse. I don't know what to do about it. And I can't imagine what's going to happen next."

I can tell Kate what's likely to happen next. Two years ago I had very similar symptoms. Like Kate, I went through several stages - feeling out of sorts, then exhausted, then drained beyond anything I'd known. And like Kate, my exhaustion was not primarily a physical thing. My work was sedentary, and I took moderate exercise. I worked from home and spent seven or eight hours in bed every night. But like Kate, something - something demonic and inexplicable - was making me wake up in the middle of the night. Like Kate, I would wake in the small hours, at two or three o'clock, and spend the next three or four hours in a state of increasing desperation. And then, when the morning arrived, I didn't wake up properly - instead, I felt as if the possibility of being asleep was gradually being removed, bit by bit, until it was time to admit defeat and get out of bed.

Then I started going beyond Kate's symptoms. I was hit by infections - wave after wave of viral bouts. Sometimes I could feel the exact moment of a virus hitting me. I'd feel flushed, and weak, and my back would ache. After a few months I had attacks that overlapped. I'd feel achy and flushed, and then develop a chest infection, and then blocked sinuses, sore ears, a painful throat, splitting headaches. At some point during this time I stopped being able to work. I couldn't concentrate. I lay on my bed. I was, in Frank Lipman's words, spent. Lying down was my new job, and it was surprisingly hard. Every day I lay there, looking up at the ceiling, wondering how long this was going to last, wondering what might happen next.

During this time I met Greg, who told me what might happen next. Like Kate, Greg would only agree to be interviewed for this article under a pseudonym. Being exhausted is not something you want to advertise. And like Kate, there was no obvious reason for his symptoms. At first, he remembers finding it hard to get out of bed. "I had no energy at all.

I ached. I had flu-like symptoms. That's what I thought it was - flu. But it seemed to be going on for an awfully long time."

Greg says: "I was living with my girlfriend, and she began to notice things. I couldn't do anything beyond work. All I could do was sit on the sofa and watch TV, or maybe read. I had no interest in going out. Moving about felt like a big effort. A half-mile walk felt like five miles. I was crushed with tiredness. Eventually I had to admit it: I couldn't work. Luckily I had medical insurance, and an understanding employer. I took six months off work on full pay."

But the medical establishment couldn't find anything wrong with Greg. He had batteries of tests. His organs all functioned well. He did not have cancer, heart disease, or even high blood pressure. Eventually, Ben Turner, a consultant at London Bridge Hospital, put him on a low dose of an antidepressant similar to Prozac. Greg says, "He said: 'Well, you may not be feeling depressed psychologically, but I can bet you that your body is chemically depressed.' He couldn't identify what was doing this to me. He said the antidepressant would act as some kind of trigger to my metabolism, to bring it back to life. And it worked. For a while, at least."

Greg gradually got better, over a period of six months. The veil of exhaustion lifted; he started functioning again. He was fine for several months. Then it happened again - the aches, the fevers, the inability to get out of bed. The zombie-like sessions in front of the TV. And what had Greg been doing? Just working in an office. "There's got to be something we're not seeing," he says. "Look at what happened to me - I felt half-dead, and the thing that brought me back to life was an anti-depressant. So this thing is partly psychological. It's generated by nothing more than normal life - a lifetime of hitting deadlines in offices, of your constantly beeping BlackBerry; But it's more than that. I would find myself in the supermarket, surrounded by bright fluorescent lighting and rivers of people, all these people arguing about what to buy. And all that stuff on the shelves. Thousands of products. And sometimes I'd look at all these people on the go, moving about, all the aggression and urgency, and I would feel completely alienated from all of it. Like I didn't want to belong at all."

One day in spring 2007 I was walking along a path beside a stream at the edge of a park. I remember looking at the path and wondering whether to cross the park or walk along beside the stream. I stopped and looked around me. I felt an ache in my back, around my shoulder blades, and a sense of unwellness creeping through me. It was familiar - I'd had two or three mild illnesses in the past couple of months.

I remember thinking: "Here we go again." I was 46.

I also remember thinking how strange it was for me to keep getting ill. For years I'd been proud of my strong constitution. Until the age of 40 I was one of those people who was "never" ill. I can clearly remember, at the age of 40, boasting that I had never been ill enough to take a day off work. At the age of 42 I had a bout of salmonella which gave me fevers and hallucinations, and this was followed by a month of other illness - bad cold, bad throat, bad cough, bad sinuses. Then I shook it off. Then I was fine for four more years.

And then came the year of the viral attacks. They were weird - not the usual 24-hour fever. More like the series of attacks I'd had after the food poisoning. There would be a fever with splitting headaches, or a fever with gastric complications. One seemed to combine everything: fever, aching muscles, splitting headache, and a sense that my bowels were about to explode - agonising abdominal pain.

Each time, I shook the virus off. But these viruses - they were closing in on me. The attacks were getting worse, and more frequent. And then, in spring 2007, as I was walking along a path, a feeling of unwellness spread through my back. "Here we go again," I thought. "I really, really could do without this." I was trying to write a book. I was behind with my work. The last thing I needed was to be out of action for a week. A week - that's what I thought it would be. The notion that it would be several months never crossed my mind.

I can look back at my life and trace the lines of what was happening to me. For years I didn't sleep enough. That was the problem. My routines were shaped by late nights, alarm clocks, coffee and alcohol. For a period in my 30s I snorted a lot of cocaine. I watched late-night TV. I watched breakfast TV. In between I fitted in five or six hours of sleep. Sometimes seven. Sometimes 10. Not a healthy routine, it turns out.

I drank in bars that didn't shut until the middle of the night. If I stayed out late one night, I would compensate the next night - by working late. Sometimes, during the day, I'd take an ice cube from the freezer and rub it around the back of my neck. That seemed to work. My coffee intake increased - from one teaspoon, to two teaspoons, to three teaspoons. I became familiar with the nauseous, anxious feeling you get when you are being poisoned by too much caffeine.

I was always doing something to wake myself up, or to keep myself awake. I would often find myself talking to somebody in the dark, in the night, not knowing who this person was. Then I'd realise that I was in bed, and after this, that the person I was talking to was a telephone operator. I always had big bills for alarm calls. During this period I could never quite trust myself around a snooze button.

When I was in my 30s, I travelled a lot. Two hundred flights in five years. I was often jet-lagged. I would land and merge into a 34-hour day, or else find that the day had already disappeared. I would usually wake myself up with a few jolts of alcohol on arrival. Sometimes on these trips I was so desperate for sleep I would find somewhere to hide from the people I was with - under a table, say, or in some shrubbery - and lie down for a few blissful moments.

In his remarkable book about sleep Counting Sheep: The Science and Pleasures of Sleep and Dreams, the science writer Paul Martin tells us what happens when we don't sleep enough. You become anxious. Your body temperature goes up. You have higher levels of adrenaline and cortisol - the stress hormones. You are more likely to develop digestive disorders - the protective protein that repairs intestinal inflammation is released while you sleep. If you don't sleep enough, you get holes in your gut.

And these things tend to be self-reinforcing. Not sleeping enough might give you stomach pain, and stomach pain, in turn, will stop you from sleeping. Sleep, says Martin, is an active state. You may not think you're doing anything when you sleep, but your brain is actually doing an awful lot. It's dreaming, for a start. If you get enough sleep, you will experience five sleep cycles, including five sessions of REM sleep - those times when your eyes flicker about and you experience surreal dreams. Each REM cycle has a slightly different function; the final one, just before you wake up, is thought to help with anxiety and depression.

So if you miss out on your morning dreams, you might become more anxious and depressed. And if you're anxious and depressed, you might find it harder to get to sleep. Lack of sleep, in turn, as Martin says, "can impair the body's immune defences and thereby make us more susceptible to infection by bacteria, viruses, and parasites". In experiments, sleep-deprived mice succumb to the flu virus even when they have been vaccinated against it. Sleep-deprived rats have bacteria in their lymph system. And something similar happens to humans. One night's loss of sleep, Martin tells us, depresses the number of white blood cells - which kill cells that have been infected by viruses - by 28%. Losing sleep also reduces the body's production of a substance called interleukin-2, a chemical messenger that keeps our immune system in shape. And then there's that pesky stress hormone, cortisol - one night's sleep loss, Martin tells us, can raise cortisol levels in the body by as much as 45%. And guess what? Cortisol suppresses immunity.

"The thing that drives chronic fatigue," Dr Ben Turner tells me, "is immunological. Your immune system misfires. I wouldn't say that it exactly breaks down, because with a lot of people it eventually gets better. In any case, the fatigue usually starts with an infection - say, a streptococcal infection of the throat, or a sub-clinical infection, something you don't even notice you've got. But if your immune system is at a sub-optimal level, that's why you get the fatigue. You get so tired because your immune system is working so hard."

Now I can see exactly what happened to me. I'm a human being living in an information-based society. Like all predators, humans tend to sleep when their prey is asleep. But information never sleeps. As a hunter of information,

I deprived myself of sleep for years. But my immune system was good, and I fought off attacks ruthlessly - until I didn't.

And then I got ill, and then the attacks became more frequent, and then, one day, I was walking along the edge of a park and felt the familiar achy feeling that signals the start of a viral infection, and for several months the viral infection did not go away. Lack of sleep had made me ill; illness interfered with my sleep. I became anxious and mildly depressed. It was self-reinforcing - chronic fatigue feeds off itself.

I lay on my bed, thinking about my immune system. It occurred to me that, until I was 40, my immune system was like the Germans on Omaha Beach, as depicted in Steven Spielberg's Saving Private Ryan. In the movie you see the landing craft bouncing through the water. The soldiers in the craft are shaking with fear, and praying, and vomiting. One guy vomits in a way that tells you exactly how terrified he is. He squirts it out, in two jerky spasms. That's how viruses used to feel when they tried to attack me.

But then what happens? The American attack is relentless. Something has to give. Tom Hanks and his little crew of men knock out a machine-gun post, and for a moment, they have "defilade" - an undefended channel they can run into. And after that, the house of cards collapses. Each virus weakens you and clears a path for more viruses to attack you.

This is what happened to me. This is what's happening to more and more people in the developed world. Everything around us - the phones and the clocks and the computers and the hand-held emailing devices - makes us busier. After a certain point we become overloaded.

"A person's total load," writes Dr Frank Lipman, "is the total amount of physical, psychological and environmental stress on his or her body. In the past 30 years this total load on the human body has quadrupled.

"My philosophy," Lipman tells me, "is that we are out of sync with our body rhythms." By "we" he means people in the developed world, which has been ruled by clocks and artificial light for more than 100 years. Human beings, he believes, evolved to sleep when it was dark and wake up when it was light. "We're also overfed and undernourished with food, and undernourished when it comes to light."

Lipman, who has written a book on exhaustion entitled Spent, is a South African who studied medicine in Johannesburg. He worked for a time in KwaNdebele, a rural area north of the city. His patients were poor, certainly - "but," he writes, "they didn't present symptoms of depression, insomnia, or anxiety." In fact, for poor people they were surprisingly healthy - "they did not suffer from fatigue, headaches, digestive problems, or the general aches and pains that my more sophisticated urban patients did."

One thing Lipman realised was that his poor patients did not have electric light - "they were forced to live with the rhythms of nature." And darkness is important, he says - in the dark, your body secretes the sleep hormone melatonin, and this in turn causes the release of other sleep-inducing hormones. Lipman himself tries to go to bed at 10 o'clock every night, in a room with no phone, TV or radio set - and, of course, no alarm clock.

"We get spent," writes Lipman, "because our modern lifestyle has removed us from nature and we have become divorced from its rhythms and cycles." And what is our modern lifestyle? Lipman is very clear on this. "We are slaves

to the corporate model," he says. "I think it is going to get worse and worse - and I don't see any improvement in the near future until we reach some kind of tipping point and wake up."

Speaking on a mobile phone from a beach in Cape Town, Lipman tells me why the modern lifestyle makes us exhausted. We're all chasing money. That's one thing.

And because we're all in debt, we have to chase more money this year than we did the last. If, say, we work in the food industry, we are forced to look at ways of producing food that costs less and that people want more - food, in other words, that is cheap and addictive... If we don't, our profits won't grow, and we'll go bust. If we do, we'll pump our customers full of processed carbohydrates and sugar, which will make their blood sugar spike and crash, exhausting them. And when they're exhausted, what will they want? More sugar and starch.

It's a snowball. An indebted economy must work harder all the time just to stay ahead of the creditors. It must spew up endless new products. More and more choice for the consumer - 50 brands of cooking oil, 200 brands of beer, 500 TV channels, tens of thousands of websites. The American sociologist Barry Schwartz has studied product proliferation, and he believes that, after a certain point, too much choice overloads our brains. "Increased choice," he writes in his book The Paradox of Choice, "may actually contribute to the recent epidemic of clinical depression affecting much of the western world." Schwartz checked out his local supermarket to see how much choice he was being offered. "There were 16 varieties of instant mashed potatoes," he writes.

In his book Faster, the science writer James Gleick makes a similar argument. "The more telephone lines you have," he writes, "the more you need... The more cookbooks you buy or browse, the more you feel the need to serve your guests something new... the more cookbooks you need. The complications beget choice; the choices inspire technology; the technologies create complication."

The modern world, then, makes us work too hard and sleep too little. It's also full of advertising, which is designed to make us feel needy and incomplete. It makes us into predatory producers and hungry consumers. It promotes individualism. It erodes community spirit. It exhausts us. That's what Lipman, Gleick and Schwartz are saying.

And there does not seem to be a way out. Madeleine Bunting, the author of Willing Slaves, a brilliant analysis of our culture of overwork, puts it simply. We live in a world, she tells me, where "nothing is enough". Driven by debt, we need to work harder all the time. Crushed by overwork, our relationships begin to break down. Robbed of the healing balm of relationships, we become more and more insecure and exhausted. More and more exhausted, we become less efficient at our jobs, which makes us more insecure, and so on. In our heads, we are always thinking about what's just ahead rather than what's happening now. "We are out of the now," Bunting tells me, "which is an exhausting place to be."

It's hard to know how many people are becoming ill with exhaustion. But it has been estimated that 100,000 Britons suffer from chronic fatigue - the worst kind. Many more feel wiped out, or spent, or so physically and mentally demoralised they can't get up in the morning. Paul Martin has collected information on how tired people are, and people in developed countries are much more tired than they should be. A poll of Americans found that 22% were "so sleepy during the day that it interfered with their activities".

In my case, tiredness became illness, which turned into a sort of malignant tiredness, which in turn prevented me from sleeping. I spent my days lying down. I did not drink, smoke or take drugs. I ate lots of fruit and vegetables, very little wheat, and no cakes and biscuits. I went on walks - at first short, and then, gradually, longer. At first the walks made me tireder. My headaches got worse. And then one day, a few months after my viral attack in the park, I walked up a hill, and afterwards I felt something novel - I felt refreshed. And I wrote my book. It's about what it's like to be middle-aged and exhausted. It's called Bits of Me Are Falling Apart.

I was lucky. Some people spiral downwards for years. As Dr Ben Turner puts it: "Some people's immune systems correct themselves, and some people's don't." Mine did. I'll be careful in the future. Early nights. No smoking. Definitely no drugs. Lots of walking. And there might be good news for all of us, in a way we never expected. "In the hectic, active world of capitalism, people pushed themselves hard towards their goals," says Ben Turner. "And now, with the recession, they might not do that quite so much."