Gorging on sweets, chocolate and anything else you can find is hardly the most spiritual way to end a day, but for the month of Ramadan it could prove irresistible. After 12 hours in which it seems that everywhere you turn someone is munching on a cake, your self-control wears thin. Which is why every night this month, millions of people will finish up with a meal that would put a rugby-club curry night to shame.

During Ramadan, the ninth month of the Islamic lunar calendar, Muslims will not eat or drink from sun-up to sun-down. And yes, to answer the question every fasting Muslim is asked, that does include water, chewing gum and smoking. Rising before dawn to eat and pray can also leave you tired and grouchy, while by late afternoon your breath smells terrible and you're ready to kill for a coffee.

Nutrition scientist Anna Denny explains that during a period without food, the body starts using up its energy reserves in a bid to keep itself working. "The body needs glucose for energy," she says. "During a fast the first thing that happens is that you use all the available glucose in your body. When your breakfast is digested, the glucose is released into your bloodstream, while the rest will be stored as glycogen. If you don't eat again, the glycogen reserves will be broken down. After this you would move on to breaking down your fat reserves and that's when people lose weight, although this is unlikely to happen in the 12 hours of the fast."

While there is no danger to healthy people who fast during Ramadan, many people still feel sluggish without regular food. Research has, not surprisingly, also linked the lack of food and water to increased irritability, changes in mood, and a lack of concentration. "You may feel headachy, light-headed and lethargic," says Denny.

Psychiatrist Anna Rahman, 29, from Wanstead, London, fasts every year, but has never lost weight in Ramadan because of the pleasures of iftar, the evening meal that breaks the fast. "Traditionally we start off with a date and water, then we pray and afterwards there will be five or six dishes of food, maybe chickpeas cooked in puffed rice or aubergines and potatoes in fried batter. Then we will have curry and rice and finally a pudding. And that's just at home - when you go to parties, there will be at least 10 or 12 dishes."

As well as getting less sleep from the early rises and late nights, eating at unusual hours can affect the way your body behaves.

Professor Tom Reilly, of Liverpool John Moores University, has studied the effects of fasting in Ramadan on athletes. He discovered that their performance deteriorated as the month went on, despite training taking place after the evening meal. He says this is because of disturbed sleeping cycles, dehydration, and the new pattern of eating.

"Your body clock affects a whole lot of functions, such as body temperature," he says. "It's not just to do with physical activity, it's also an internal mechanism. During a fast you take energy in at night, when you normally are slowing down. If you fast, you have to accept that there will be a deterioration of performance, not just for athletes, but for all workers. We have found that performance does not just go back to what it was, but takes time to adjust."

So if fasting does all this, why do major religions advocate it? On Friday, Jews will be undertaking the 25-hour fast of Yom Kippur to ask God's repentance for their sins. Orthodox Christians, meanwhile, can spend up to half a year in various forms of fasting, which is believed to bring them closer to God.

Fasting is often seen as a way of instilling self-control and used as a time of reflection. For Muslims, Ramadan is a time to celebrate the revelation of the Qur'an to the prophet Muhammad, and a time to reconnect with your faith and community. Instead of giving in to your moods during the fast, anger, greed, gossiping and envy are as out of bounds as a burger or Coke at lunchtime.

Father Brendan Callaghan, a lecturer in the psychology of religion at Heythrop College at the University of London, believes restricting what you eat can have a positive effect on your mind, by creating a break in your normal routine - as long as you are undertaking it with that aim in mind. "Most religions have found that fasting takes the mind away from everyday things, and so you become more aware of God," he says. "There's a measure of control and renunciation involved and it is something that is not your normal way of life, which makes it special. In short-term fasts, at least, there's some evidence in psychology and medicine that it helps people think more clearly."

NHS workers are now even hoping to harness the self-control shown during Ramadan to make Muslims healthier. Trusts throughout the country have launched campaigns to help people give up smoking permanently, exploiting the month when their willpower is particularly high.

For most Muslims, the emphasis on prayer and reading the Qur'an leaves more time for reflection and creates a sense of calm, says Rahman. "It's amazing, and really important to me," she says. "You feel especially close to your family and part of a big community when you break your fast together. It gives you time to think about your relationship with God and the kind of person you want to be."

Sunita Wallia, a specialist community dietician, advises people on how to keep fasting as healthy as possible. Although many people find it hard to wake up for the morning meal, or sehri, Wallia warns that eating only one large meal a day can be bad for your lipid profile - the amount and type of fat in your blood. Instead, she suggests eating a breakfast of wholegrain cereals, such as porridge or muesli with chopped fruits, nuts and seeds, or granary bread. These are low glycaemic index foods, which slowly release energy to keep hunger pangs at bay for longer.

While bingeing at night may seem impossible to avoid it can make it harder to sleep and to fast the next day. Instead Wallia advises enjoying some treats, but trying to stay away from fried food and remembering that most people exercise less in Ramadan and so need fewer calories than normal.

Adding vegetables to meat dishes, having a salad with the meal and watching the fat and salt in preparing traditional dishes can all help, along with taking a 20-minute walk after iftar (the evening meal).

Yet, despite her good advice, she knows that it is hard to keep people away from a table filled with delicacies. She says: "I have heard from people that what they would have in six months they end up eating in one month, because they look forward to it so much."