If you've overdone it on the cookies and free wine at the Christmas party, you might wake up deciding to hit the gym to make amends. But you'd actually be way better off taking it easy, rather than trying to sweat all that cheap booze out on a treadmill.

Even if you feel OK, heading to the gym after a night of drinking isn't necessarily the best idea. Why not? Because alcohol changes the way your body works so the usual benefits of exercise aren't in place while your body is still trying to process booze.

Keith McNiven, a personal trainer and founder of Right Path Fitness, told The Sun that the concept of "sweating out the alcohol" is a myth we've swallowed hook, line and sinker.

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Here are four reasons why you're better off staying in bed than hitting the gym:

You're dehydrated

The main cause of a hangover is dehydration, meaning that if you work out without hydrating yourself properly, you can actually make yourself feel worse.

"The alcohol from the night before will have dehydrated your body, and exercise will only exacerbate it," Keith explains. "That can lead to a reduced performance on your part, and depending on how much alcohol you’ve had, odds are also that you still have it in your system the next day — it’ll take you a long time to burn off. (The average person burns off around one unit per hour, depending on circumstances)."

Lucy Arnold, personal trainer and founder of Lucy Locket Loves, agrees and suggests hydrating well before attempting a gym session.

"Alcohol causes the body to lose water and you need to replace that water, especially if you're going to get hot and sweaty," she says."

"Make sure to drink water and get yourself hydrated before even attempting to exercise, if you don’t want to feel worse later in the day."

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You're at high risk of injury

Keith says to stay away from weightlifting, in particular.

"Your coordination will be off, and depending on your level of dehydration (e.g. if you’re feeling dizzy), you could hurt yourself pretty badly and strain your muscles.

"Stay away from hot exercises too, such as Bikram yoga, and skip the sauna after your session. If you really can’t resist, try a few simple floor exercises, light stretching or swimming, and drink plenty of water throughout the entire day.

"Ideally, just give yourself and your body a day off, and do a few extra reps the day after instead."

HE SAID TAKE A DAY OFF!

HE SAID TAKE A DAY OFF.
(iStock)

You can't burn fat or build muscle

"You've got 75 percent less muscle synthesis — so you're not going to build any muscle really… and you've got about 75 percent less fat oxidation too, so you're not going to burn fat either," says Laurence Fountain, founder of Salus London.

Working out on a hangover interrupts the muscle gain and recovery, adds Keith.

"You're not working as hard as you normally would because your body is already focused on getting rid of the alcohol and there's decreased blood flow in your muscles (again, because you're dehydrated)," he says. "You'll burn fewer calories too, because the alcohol is preventing your body from using its natural energy source: glucose."

"While alcohol isn’t stored as fat," Lucy adds, "it blocks fat oxidation which basically accelerated the rate at which your body stores fat as body fat."

And you're sleep deprived

"On top of all of that, you've got the fact that your sleep quality is always poor after drinking alcohol," Laurence tells the Sun. "Your brainwave cycle is completely different under the influence of alcohol, so you're going to be in a sympathetic state."

That means that the food you eat in the morning won't be in place yet, your amino acid pool is depleted, and you'll be lacking the vital nutrients to function at a reasonable level.

"It's just a really stupid idea." he adds.

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By all means, go on a walk to get some fresh air and get your blood pumping a bit — but there's no point in actively trying to burn fat or build muscle.

Stay hydrated, pop out to the corner store for any beverages or vitamins, and get back into bed.

This story originally appeared in The Sun. Read more content from The Sun here.