Next time you wake up in the middle of the night with a craving for cream buns or chocolate, don't feel down. You're not a greedy pig. Your body has simply demanded more andoginous optic peptides

Food cravings tend to be thought of as mere nutritional foibles - or a sign that you lack willpower. But did you realise that a craving for ice cream may signify the need for a career change? That steak cravings may explain why you're exhausted? Or that that desperate need for daily bagels may mean you should book a medical check? According to many nutrition experts, food cravings are our bodies' way of telling us something - and that we should learn to listen to them.

We've all been there. Having not eaten red meat for months, you suddenly get a desperate hankering for a burger or a lamb chop that just won't go away no matter how many chicken dishes you try in order to satiate it. Or, come 8pm on a Wednesday, nothing will make your life complete but a pot of Ben and Jerry's Chubby Hubby - and unless you get it, your mind can't concentrate on anything else.

This is a craving. "It will have hit fast and it will have hit furiously," says Dr Adam Drewnowski, head of the Nutrition Sciences Programme at Washington University in the US. "It differs from hunger or appetite, where you just need to eat food, in that it's specific. You know exactly what you want to eat, sometimes even down to the brand name and flavour, and you have to have it - nothing else will do."

But why? Well, all the experts agree that food cravings do tell us something but they differ on exactly what these messages are about.

One school of thought believes that cravings are triggered purely by a nutritional need. Something is missing in our diet and those "I'd-kill-for-a-KitKat" urges are your body's way of revealing it. "The most obvious thing that a craving identifies is that your body needs energy," says Harley Street nutritionist Kate Neil.

Our energy levels are kept in balance by the fluctuation of our blood sugar levels. These are raised by eating and they gradually lower as time passes. When they start to bottom out, our bodies send signals to the brain and we get hungry. If, however, the blood sugar falls suddenly (which can happen after a big lunch or an exercise session), the brain panics. It wants to restore the levels fast, so instead of just sending out a hunger signal it relays a command for something sweet or starchy which it knows will raise blood sugar levels fast. This is the most common reason for those 4pm sugary snack attacks (it also accounts for the PMT chocolate frenzy, as the variations of hormones pre-period raise sensitivity to blood sugar changes). However, if what you're craving isn't sugar coated, it could reveal a more fundamental dietary deficit.

Every food we eat provides some level of calories and nutrients and one way our body regulates appetite is by weighing up the ratio of these to each other. If you eat a meal that is high in nutrients for the calories it provides, your body will be satisfied. It will then only trigger eating again when the sugar balance alters. However, if the ratio is one of high calories and low nutrients, the body will even things up by craving foods again. Exactly what these cravings are depends on what you have or haven't eaten so far.

As omnivores, humans are designed to get our food intake from a variety of different nutritional sources. The healthiest diets contain at least 16 different foodstuffs every three days. Dr Drewnowski says that if we eat a less varied intake, our body realises it and "tries to shake us up a bit by stimulating an urge for the foodstuff that is missing. If you've been living on junk food for three weeks because you're busy, all of a sudden you'll wake up desperate for veggies or salad. Conversely, someone who rarely eats red meat may, every so often, find they're craving steak."

However, some experts think the variety approach is simplifying things. That rather than just increasing our general nutrient content, cravings are aimed at replenishing a specific vitamin or mineral that we lack. "They're like an evolutionary alarm system," says Antony Haynes, from London's Nutrition Clinic. "Our bodies are incredibly sophisticated and know that to be in perfect health they need to be in perfect balance. Over the years, it develops a recall system of how foods make us feel and what they provide. If you become deficient in a particular nutrient, it calls on this memory bank to determine what normally provides you with that and, bingo."

As a result, Haynes believes that if you are lacking iron, for example, you'll crave red meat; if you lack fatty acids, you'll get cheese urges; a hankering for salty foods, such as sardines, comes from a lack of minerals. Yet most of us don't crave meat, cheese or sardines - rather ice-cream, chocolate, pizza or tortilla chips. "And you don't crave these because you're deficient in any vitamin they contain," says Dr Drewnowski. "These provide an intake of one of three things - fat, sugar or salt - and these don't fulfil any nutritional need. It's neurological."

Research has shown that when we eat a food containing sugar, salt or fat our brain triggers chemicals called andoginous optic peptides that are similar to hormones called endorphins. These are like aspirin for your emotions, making you feel happier, calmer and more able to control feelings. During a period of stress, depression or even just after a bad day at the office, your body craves these endorphins but they aren't easy to get. AOPs, on the other hand, can come with a mouthful of crisps or chocolate and the urge is stimulated.

So how do you tell if your craving is nutritional or neurological? First, look at what you're craving. Then look at how much of it you're craving. A nutritional craving is normally satisfied with one spoonful of ice cream or one piece of steak. "If you have to eat the whole tub or crave the entire cow, you're feeding a psychological urge," says Dr Drewnowski. "Once in a while this is fine, but if it's happening regularly, I'd say look at what's going on in your life to cause that and fix it."

But what if you find yourself craving foods daily yet everything is okay in your life? This is when the third theory kicks in - that cravings may be your body's way of alerting you to an allergy or intolerance. It's believed that only 25 per cent of the population can eat the high levels of grains and dairy products that exist in today's diets without some kind of negative reaction. The other 75 per cent of us have the potential to develop an allergy and intolerance and exposure to the trigger foods can lead to problems with digestion, the skin and our energy levels. "And if someone is intolerant to a food they often crave it," warns Kate Neil.

The reason is that the presence of an "intolerant" food in the body causes a reaction almost like a high. First, it sends blood sugar (and energy levels) soaring and can create substances called exdorphins which mimic the pleasurable effects of the endorphins we mentioned earlier. However, once these reactions have worn off, the body is left "depressed" and low in blood sugar - so it sends out a message for the "fix" it got earlier and you crave the bread, milk, cheese or whatever. "Allergy cravings are tricky because often people don't think what they're doing is bad," says Ms Neil. "After all, if you get the sudden urge for a slice of bread or glass of milk that's good because they're healthy foods. But if you're intolerant you could be causing problems by giving in."

Ah yes, giving in. The eternal question. Should we or shouldn't we? Well this is one area where all the experts agree. "If you can control the craving rather than it controlling you, then go for it," says Dr Drewnowski. "Cravings are only a problem if they are more like binges or habits. Then you should be looking at the real reason behind them and deal with that."

That's where the following tips should help:

Eat little and often and mix protein and carbohydrates at every meal. "This will regulate blood sugar levels decreasing the risk of sweet cravings," says Kate Neil.

Vary your diet and take a multi-vitamin. "I haven't had a client yet who hasn't felt cravings decrease once they're taking in adequate nutrients," says Antony Haynes.

Don't feed mood swings. Tackle the problem not the symptom.

If you feel you "have" to have a food every day and you suffer other symptoms such as fatigue, digestive problems or skin complaints, consider seeing a nutritionist to rule out intolerances.