Sometimes I worry in the night about going blind, or infertility, or getting cancer or Aids. At other times, it’s about things I shouldn't have said, phone calls I should have made, things that need sorting out and that I know I’ve been ignoring because I've got too much to do,” says Kate, 29, a lawyer. During the day, she seems the picture of cheeriness and calm but, like many others, when she wakes at 4am, she becomes strangely low.

Kate, it turns out, is speaking for about one in five of us, for that is the proportion of people who wake during the night or early in the morning — often at 4am on the dot — feeling jittery and with our minds whirring with thoughts, usually negative ones.

Traditionally, early-morning waking has been linked with depression, but sleep expert Dr Neil Stanley of Norfolk and Norwich University Hospital believes that more and more of us — as many as 20% — are suffering from waking and worrying in the night, not because we are depressed, but because modern life is increasingly tense.

Clinical psychologist Linda Blair agrees. “More than 80% of people who have anxiety or depression have insomnia, but insomnia is not necessarily a sign of depression,” she says. “We’re working around the clock, waking up to never-ending to-do lists, childcare hassles and interest-rate fluctuations. People are on the alert for disruption and stress at all times.” Even, it seems, when we should be getting some shut-eye.

Some research puts the number of people suffering from sleep problems even higher: a 2007 study by the National Sleep Foundation (NSF), in America, found that almost two-thirds of women report one to three disturbed nights’ sleep each week. Interestingly, this also appears to be more of a female problem: women are almost 20% more likely to suffer from insomnia than men, according to the NSF — perhaps because of all the stressful demands of work and family that make the modern woman’s life so fraught.

When this started happening to me about a year ago, I assumed I was in a minority, until I began to talk to my girlfriends. Nearly all of them were getting the 4am blues, too. But why? Research suggests our sleep patterns are determined by our genes; about 10% of us are morning larks and 10% night owls, with the rest of us somewhere in between. You have as much chance of changing this as you would changing your eye colour. Those of us who get the 4am blues probably went to sleep around midnight, and, struck by feelings of pre-dawn anxiety, we know that if we don’t get back to sleep, we’ll start the day feeling shattered.

In fact, we wake naturally many times during the night, as each sleep cycle of about two hours ends. Plus, our body clocks aren’t programmed to make us feel sleepy and alert only twice in each sleep cycle. Animals — and human babies — often naturally sleep and wake many times in a day; and in hot countries, it’s normal to sleep for five hours at night and three in the afternoon as a siesta.

At night, the body normally shifts from deep sleep to the rapid eye movement (REM) phase of light pre-waking sleep from 3am onwards. REM sleep prompts the body to wake up, so it is normal to have up to 60% of REM sleep in the second half of the night.

If we are already wound up, we become more sensitive to this internal wake-up signal — as we are to other signals such as rising body temperature and falling melatonin levels.

Once we’re in the pre-waking phase, signals in the environment can also easily disturb sleep — a husband snoring, the cat jumping on the bed, needing the loo, early summer dawn breaking or being too hot (for instance, during menstruation, a woman’s body temperature rises and she is more likely to wake).

Whatever the reason for waking, once we are awake, says Stanley, we’ve probably met most of our sleep needs, so the body doesn’t see any reason to let us go back to sleep. As we age, our need for sleep also decreases, but that shouldn’t really affect women until their late forties. “However,” he says, “you tend to have more responsibilities in your thirties and therefore naturally have more worries — you’ve probably got a mortgage and are wondering how you’re going to be able to the pay bills.”

As to why we often think negative thoughts when we wake in the early hours, that is entirely psychological. It could be because we’re cut off from familiar stimuli. It’s dark and quiet; you’re trying not to move so you don’t wake your partner, and you have no way to communicate.

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