* A medium-strength cup of instant coffee contains around 70mg of caffeine.
Espresso has about 100mg in a single shot; a mug of strong filter coffee can contain as much as 175mg. Tea has around 15-30mg per cup
* Medical consensus is that 400mg of caffeine is a sensible daily limit
* You can die from caffeine overdose, but you'd need to drink 75 cups of coffee in a day
* European consumption of fruit and herbal teas increased by almost 50 per cent between 1997 and 2002
* Chocolate biscuits really do give you a lift. The sugars are quickly broken down and enter your bloodstream soon after eating, which means a sudden release of energy and a mood boost
* But your body responds to the sugar shot by producing a surge of insulin, which clears the glucose from your bloodstream. Before long you're back where you started
In some jobs it's possible to ride out a mid-afternoon energy slump with a bit of mindless paperwork. Teachers don't always have that option. So what can you do when you feel your energy levels flagging? Reach for a strong cup of coffee and a fistful of chocolate biscuits, perhaps. But are caffeine and sugary snacks useful refreshments or hazards to your health?
In some staffrooms it's organic and fair trade; in others it's industrial and low-grade. Either way, coffee is the fuel of the teaching profession: a search of the TES online staffroom reveals more than 5,000 posts containing the word coffee, as opposed to just 900 mentions of textbook. So what's the big attraction? The fine aroma of ground beans? Perhaps. But for tired teachers the stimulant effect of a shot of caffeine can be just as welcome.
A medium-strength cup of instant contains around 70mg of caffeine, equal to two strong cups of tea. Espresso has about 100mg in a single shot. Filter coffee, meanwhile, is less potent than espresso, but is usually drunk in larger measures, meaning a mug of strong filter coffee can contain as much as 175mg of caffeine.
Full of beans?
Caffeine in coffee stimulates the central nervous system, the cardiac muscles and the respiratory system. Scientists believe that caffeine improves alertness by hindering the action of adenosine, a neurotransmitter which has a calming effect on the nervous system. This explains why drinking coffee can stave off tiredness, but it may also explain why some people feel anxious or uptight if they drink too much.
But while some people get fidgety after just a few cups, or find it hard to sleep after a late-night espresso, others can drink as much coffee as they like. The short-term effects of caffeine vary widely. Smokers may find that the effects only last briefly, while women on the Pill may find it stays in their system for half a day or more. Research at Bristol University in 2004 suggested that caffeine raises stress levels in men, but not in women. It may partly be to do with our state of mind when we reach for the coffee jar. "Caffeine heightens awareness," says Dr Sarah Jarvis, a GP in Hammersmith, west London, with a keen interest in the coffee debate. "If you have feelings of anxiety you become more aware of them, but that isn't necessarily the same thing as the coffee itself making you anxious."
Is caffeine bad for you?
It's possible to die from an overdose of caffeine. But you'd need to down 75 cups of coffee in a day, which is probably beyond even the most hardened espresso addict. At the other end of the scale, it's generally agreed that one or two cups a day won't hurt. "There's no harm at all in using coffee to get yourself through a low point in the day. It's safe, and it works,"
says Dr Jarvis.
The debate centres on whether moderate or heavy coffee drinkers - those who drink more than four cups a day - are putting their health at risk. The consensus among medical experts is that 400mg of caffeine is a sensible daily limit, or 300mg for pregnant women. With this kind of intake, long-term risks seem to be negligible. Claims that tea and coffee cause dehydration have been challenged by two recent studies which suggest that the diuretic effect of caffeine is mild, and more than balanced out by the water content in the drink itself. Coffee can raise cholesterol levels, but only if the beans are boiled and drunk without being filtered, a method common in Scandinavia, but not in the UK. And from time to time, studies have linked coffee drinking to an increased rate of cancer - particularly pancreatic cancer - but there's no real body of evidence and most scientists are sceptical. Research is complicated by the fact that many heavy coffee drinkers are also smokers, big drinkers or have a poor diet.
Some studies have even suggested caffeine can be good for us, helping asthma sufferers and alleviating the symptoms of common colds. And a visit to the website of the Coffee Science Information Centre - run by a conglomeration of European coffee marketing boards - claims that as well as boosting alertness and concentration, caffeine can improve sperm motility, protect against type 2 diabetes and help prevent Alzheimer's and Parkinson's diseases.
How about decaf?
People who are sensitive to the stimulating effects of caffeine often choose decaf as an alternative: the decaffeinated coffee market in the US is worth around $500 million (pound;266 million) a year. Sales have gradually declined, however, over the past five years and not everyone is convinced that decaffeinated tea and coffee are healthier than the real thing. Some experts are concerned about the use of solvents in the decaffeination process, while a study in the US last year concluded that because much decaffeinated coffee is made from robusta beans, which have a higher fat content than standard arabica beans, it could potentially raise cholesterol levels.
Turning over a new leaf?
Tea has less caffeine than coffee, around 15-30mg per cup. But don't be fooled by the colour: caffeine content is much more closely linked to the way the leaves have been prepared, and teas such as the green Japanese gyokuro contain far more caffeine than much darker teas such as lapsang souchong, which has very little.
Even the weakest teas, however, still have enough caffeine to have a noticeable effect on the central nervous system. And many people find tea just as uplifting as coffee, probably because it contains several other stimulants, such as theobromine. Like coffee, tea has been linked to protection against diseases, particularly ovarian cancer, and contains a variety of antioxidants. It also contains fluoride, which is good for the teeth.
Let's get fruity
European sales in recent years have reflected the shift away from traditional black tea towards fruit and herbal teas, consumption of which increased by almost 50 per cent between 1997 and 2002. Green tea, too, is becoming more popular: consumption in 2002 was more than 20 times higher than in 1997.
Most herbal infusions and fruit teas provide a caffeine-free boost, with ginger and ginseng among the most ancient and commonly championed energy pick-me-ups. Ginger stimulates circulation and improves blood flow, which make you feel more energised. The active ingredient in ginseng is a nutrient called Ginsenocide, said to combat fatigue. Ginsenocides stimulate the nervous system and have strong antioxidant properties, and work as anti-inflammatories and regulators of blood sugar, cholesterol and blood pressure. As well as giving an energy boost, ginseng seems to have a built-in feel-good factor: it promotes the release of a pituitary hormone known as adrenocorticotropic hormone (ACTH), which helps counter stress.
Whichever tipple you choose, the psychological benefits of a hot drink and a sit down may be more important than any antioxidants or minerals. A study carried out in California last year, for example, found a markedly lower suicide rate among tea and coffee drinkers than among those who abstained.
It's good to graze
If you feel you need a more substantial boost than a quick cuppa, it may not just be the snacking gremlins leading you into temptation. While some dietitians still suggest sticking to three square meals, and resisting the urge to eat in between, most nutritionists now agree that eating between meals can be a good thing. The body's main source of energy comes from glucose, more often called blood sugar, which is broken down and then used for everything from muscles to the brain. If blood sugar levels drop, it can be hard to concentrate on marking work, and even harder to teach a dynamic lesson. Fluctuating blood sugar levels are obviously a problem for people with diabetes, but everyone is likely to feel better if they can keep their blood sugar on a reasonably even keel. Which is where the snacks come in. "You want to be bobbing along on a sailing ship, not riding a roller-coaster," says Teresa Dupay, a nutritionist who works in schools.
Some scientists even argue that snacking is a human instinct, pointing out that our cave-dwelling ancestors probably had no regard for meal times, but ate when they felt hungry or when food was available. Unfortunately, in the modern world, much of the food we associate with snacks is high in salt, sugar or fat. It's not snacking itself that's unhealthy, it's the type of food we choose to snack on.
So pass the biscuits
It's not all in the mind: a chocolate biscuit really will give you an energy lift. The sugars in biscuits, cakes and chocolate, also known as simple carbohydrates, are quickly broken down by the body and enter the bloodstream soon after eating, which means a sudden release of energy and an accompanying lift in mood. Enough to get you through a tricky class, or a pile of marking.
A square or two of dark chocolate might be even better. While it contains less sugar than most "chocolate bar" products, it can contain other stimulants such as caffeine and theobromine, a stimulant found naturally in cocoa beans. Good-quality dark chocolate also increases the production of serotonin, a natural antidepressant, and contains antioxidants and a feel-good chemical called phenyl ethylamine.
A roller-coaster ride
But tucking into the biscuit tin may, in the end, make your energy slump worse. It's not just that high-fat, high-sugar foods tend to be low in the nutrients needed for long-term health; the buzz you get from the morning munchies may be the reason you can't get through afternoon classes without a boost.
The body responds to the sugar shot by producing a surge of insulin, which clears the glucose from the bloodstream. Before long you're back where you started, craving another hit of sugar. And when blood-sugar levels fall rapidly, the body often releases the stress hormone, adrenaline, making you grumpier than ever.
Complex carbohydrates provide a slower release of energy, so you don't suffer the same roller-coaster effect. That's why carbohydrate-rich energy bars, or a bowl of cereal, are a better alternative than most sweets and chocolate, though sceptics reckon they're no more effective than eating a couple of bananas.
If you can avoid sugar rushes altogether, then the high and lows should be replaced by a tranquil plateau of well-being. "If you eat healthy foods that release energy slowly, then you'll find you feel a lot calmer," says Teresa Dupay. "It will help you cope much better with potentially stressful situations."
Going bananas: the healthy alternative
Pride of place for healthy refuelling probably goes to bananas. As well as being convenient to eat and packed with vitamins and minerals, their sugars are bound up with plenty of fibre to ensure a steady release. But replacing biscuits and chocolate bars with fresh fruit and nuts requires a fair amount of willpower. It's easier to make the change if your energy levels don't drop too much in the first place, which means your main meals need to be substantial.
"Breakfast is a must," says Teresa Dupay, who recommends a morning meal of complex carbohydrates, such as porridge, followed by nuts or seeds and possibly some fruit. "And if you have breakfast at seven," she adds, "don't be surprised if you need another piece of fruit before school starts."
Refreshing yourself with elevenses, so that you can skip lunch to run the five-a-side kickabout, is never going to wean you off the sweet stuff.
Taking time out for meals, and then taking time to enjoy them, helps digestion, boosts mood and may mean you don't need the staffroom pick-me-ups after all.
"Too many teachers eat lunch on the hoof," says Ms Dupay. "If you eat in a hurry you won't absorb the full benefit of the food."
* UK Tea Council (www.tea.co.uk)
* British Coffee Association (www.britishcoffee association.org) and (www.coffee-break.org)
* Coffee Science Information Service (www.cosic.org)
* British Dietetic Association (www.bda.uk.com)
* The British Heart Foundation (www.bhf.org.uk)
* Food Standards Agency (www.food.gov.uk)
* Teacher Support Network (http:www.teacher support.infoindex.cfm?p=3507)
Main text: Steven Hastings
Photograph: Dorling Kindersley
Additional research: Sarah Jenkins
Next week: Sexually transmitted infections