The average Brit* apparently spends the equivalent of 315 days of their life with a hangover - now there's a sobering though on New Year's Evet!

But what's actually going on when we're hanging after a heavy night?

You wake up with a banging headache, dry mouth and your tongue seems to have grown a carpet. You also have blood-shot eyes and find noise intolerable

"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.

"Your eyes dry out, so there is less fluid to lubricate the eyeballs. And you generally become oversensitive to noise when your head hurts."

Details of the night before are hazy. You also feel down in the dumps, or even depressed and full of dread

It's not uncommon to experience some degree of memory loss, particularly after a very heavy drinking session.

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

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As for our moods, while we may get a buzz and feel perkier after a couple of drinks, alcohol's actually a depressant, affecting neurotransmitters - brain chemicals - and possibly resulting in us feeling angry, teary and even depressed and anxious for days and weeks afterwards.

You can't stop yawning and have difficulty concentrating

When you're extremely hung-over, 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.

As above, all that brain chemistry disruption may play a part in why you're not feeling sharp, but another big factor is the poor night's sleep you've just had.

"Firstly, if you have a lot to drink, you may need to get up in the night to go to the toilet,” says Christina Merryfield, lead dietician at Bupa Cromwell Hospital.

"Secondly, 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.

"Finally, drinking also relaxes muscles, so although you can feel relaxed, you are more likely to snore loudly, causing yourself to wake up!"

You've got diarrhoea and might be feeling queasy and throwing up

"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."

[Related story: Beauty tricks to hide your hangover]

Feeling - and being - sick can also be due to a high concentration of alcohol in your stomach and bloodstream; especially bad if you've been mixing your drinks.

Stomach acid changes can also cause heartburn, and may explain why you've got the runs - though some people, especially those with irritable bowel syndrome (IBS), may also be sensitive to the sugars in certain drinks.

Limbs feel heavy, tired and sluggish

OK, so you were giving Jacko 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 the booze you were glugging will probably have played a part too.

Yet again, dehydration has a lot to answer for. "Loss of fluid in the body affects the blood flow through all of our body tissues," Wakemen points out, 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.

*Macmillan Cancer Support carried out the survey to launch their Go Sober for October fundraising campaign, which involves getting people to sponsor you not to drink a drop of booze for the month of October.