You're exhausted, everything hurts and all you want to do is to have a fry-up in front of re-runs of Friends.

Apparently we spend almost a year of our lives hungover. So what's actually going on inside our bodies when we're hanging after a heavy night?

Head, ears and eyes

You wake up with a banging headache, dry mouth and blood-shot eyes. You might find noise intolerable too.

"All due to dehydration," says Mel Wakemen, a senior lecturer from Birmingham City University's Faculty of Health. "Alcohol is a diuretic [makes us pee a lot], so our body essentially becomes dry. Headaches can be caused by this, as blood flow to the brain changes.

"Alcohol may also damage the sensitive parts of the inner ear that help us hear sounds, and can lead to deafness."

Mind

Details of the night before are hazy. You also feel down in the dumps, or even depressed, and full of dread. And obviously it's not uncommon to experience some degree of memory loss.

"Alcohol affects brain cells and stops them from storing information in our memory bank, ie. stops us making memories, particularly short-term memories," says Wakeman.

It can affect long term memory too. A study published in the American Journal of Geriatric Psychiatry this summer assessed the mental abilities of almost 7,000 people across eight years.

They found those with a history of drinking problems had a more than doubled risk of severe memory impairment further down the line.

Mood

While we may get a buzz and feel perkier after a couple of drinks, alcohol is of course a depressant, affecting neurotransmitters - brain chemicals - and possibly resulting in us feeling angry, teary and even depressed and anxious for days and weeks afterwards.

Alertness

When you're extremely hungover, even getting from bed to sofa can feel like an achievement, and watching re-runs of your favourite comedy is the most your brain can handle.

Basically all that brain chemistry disruption is the reason why you're not feeling sharp, but another big factor is the poor night's sleep.

"Alcohol can disrupt sleep for a number of reasons," says Christina Merryfield, lead dietician at Bupa Cromwell Hospital.

"A deep sleep helps the body to restore itself, but alcohol can affect the initial process needed for deep sleep by interfering with the first stage of sleeping, called 'rapid eye movement' [REM]. This disruption may also contribute to making you feel drained when you wake up."

Gut

"Alcohol can upset your stomach by raising your stomach acids, which causes you to feel nauseous and unwell," says Merryfield. "This usually lasts 24 hours, but can be longer if you've drunk excessively.

"[Feeing sick is] our body's way of protecting itself - by making us feel rough, it's giving us some aversion therapy to stop us doing it again!"

Stomach

As for those cravings for stodge and sugar, that's largely linked to low blood sugar.

"Many alcoholic drinks are rich in carbohydrates, making our blood sugars surge upwards, followed by a downward crash as our body attempts to regulate our levels," says Wakemen. "We often get the munchies or feel hungry in response."

Drops in blood sugar are also what cause some people to feel shaky, weak and dizzy, Merryfield points out, which is why it's "important never to drink on an empty stomach".

Arms and legs

OK, so you were giving Justin Timberlake a run for his money on the dance floor until the early hours, and that might have something to do with why your arms and legs are aching so much, but it's mainly due to dehydration.

"Loss of fluid in the body affects the blood flow through all of our body tissues," Wakemen says, so this includes all your muscles and connective joint tissues. Low blood sugar might also be a factor, as you'll generally have less energy all over.

Immunity

If you feel run-down all the time, and keep getting colds when you've been burning the candle at both ends, it could be down to boozing. Disrupted sleep can affect the body's ability to fight off infections, as can dips in nutrition.

"Alcohol can reduce the absorption of some nutrients, so if you drink over the recommended amount on a regular basis, in time, a reduction in nutrients, along with damage to your liver and other organs, will reduce your ability to fight off infections and you'll be more prone to illness," says Merryfield.

Macmillan Cancer Support have launched a Go Sober For October fundraising campaign.