Happy New Year and welcome to 2017!

Many of us may not be feeling the welcome just quite yet. After all, Newton's third law states that for every action there is an equal and opposite reaction.

And while he was talking about physics, not biology, it certainly feels like hangovers follow that law, though we might rephrase it as "for every moment of inebriated exhilaration, there is an equal and opposite feeling of pain and unease."

But what is it about throwing back a few too many that leaves you feeling only halfway human: your head throbbing, your mouth dry, your stomach on edge? Could that feeling really be explained by dehydration, as so many seem to think? Even more important, is there a cure to hangovers?

Unfortunately — spoiler alert — the answer to the cure question is no. And as for dehydration, it's not the main culprit, hangovers are more complicated than that. In fact, much about hangovers is still a mystery to scientists. But here's what we know.

And just a note: We're looking at hangover symptoms specifically here, not the effects of long term alcohol abuse. Frequent hangovers could be a sign that you should consider cutting back on drinking.

A major component of hangovers has to do with the way our bodies break down alcohol.

The way we metabolise alcohol is at least partly responsible for hangovers, according to Richard Stephens, a psychology professor and member of the Alcohol Hangover Research Group, an organisation started to answer questions in what they call the "neglected issue" of hangover studies (unfortunately, it doesn't seem they've made much progress).

Stephens told The Atlantic in 2014 that one thing researchers know is that our bodies first metabolise ethanol, the main alcohol in booze. But we break ethanol down into other alcohols, including methanol, which our body turns into formaldehyde and formic acid — toxins that make you hurt.

This process happens about 10 hours after we stop drinking.

Other byproducts of alcohol might be responsible for part of the pain.

We also produce acetalydehyde when we break down booze.

We break that down afterwords into acetate, which is harmless, but acetalydehyde is known to cause vomiting, nausea, a flushed face, and sweatiness — making it a prime culprit for at least a few hangover symptoms, according to some researchers.

But others have found that hangovers aren't the worst when acetalydehyde levels are highest, meaning they probably aren't responsible for everything.

Some types of alcohol really do lead to worse hangovers than others.

Whiskey might be your drink, but there's some truth to the idea that alcohol with more congeners (a chemical component of booze that's more often found in dark drinks like bourbon, red wine, or dark rum) can lead to a worse hangover.

One study titled "Intoxication with bourbon versus vodka: effects on hangover, sleep, and next-day neurocognitive performance in young adults" found that whiskey did indeed lead to more severe hangovers.

It's not all bad though. One of the authors of the study told Scientific American that some of those congeners in whiskey helped "protect the stomach lining from damage."

Drinking dehydrates you, and that dehydration usually accompanies a hangover but probably isn't responsible for it.

It's common wisdom that hangovers — and the accompanying headaches — are due to dehydration. People swear by chugging a jug of water before bed, or for more intense pain, Pedialyte Freezer Pops, specially designed to rehydrate children suffering from severe dehydration.

But most researchers say that the dehydrating effect of alcohol is exaggerated, though still real. At least some of those dry mouth symptoms (and definitely headache symptoms) come from the break down of alcohol — mentioned above — not dehydration. And while we need water in our bodies to break down alcohol, we still won't feel better until our systems have dealt with the byproducts of that process.

If your hangover is bad enough that you are vomiting or have diarrhea, definitely rehydrate — using Pedialyte as needed. And go ahead and do it if it makes you feel better anyway. But don't expect a magical cure.

There's a biological basis for the idea that "hair of the dog" helps — but that also explains why hangovers are a risk factor for alcoholism.

(Getty Images) (Getty)

If someone has a drink the next morning, their body will soon realise that there's more ethanol in their bodies to start breaking down. Since our bodies prefer ethanol, they'll stop breaking down methanol into those toxins at this point, which is why a bit of the "hair of the dog that bit you" can at least temporarily take the pain and sick feeling of a hangover away.

But really, that's just putting off the pain. Your body still has to break down the ethanol.

Stephens says that researchers think this "hair of the dog" effect is why hangovers may be a risk factor for alcoholism instead of a natural deterrent to becoming an alcoholic. Studies show that alcoholics get some of the most severe hangovers around.

Hangovers actually may get less severe as you get older.

This goes against conventional wisdom and may seem impossible — lots of people recall being able to party and study back when they were in college.

But a study in the journal Alcoholism of 51,645 Danish men and women found that the older someone gets, the less likely they are to experience a severe hangover after a binge drinking session — even after controlling for food consumption and quantity and frequency of regular drinking.

Our bet? You might just physically feel worse in general if you are older and less fit than you once were. Plus, you might have more responsibilities that force you to get off the couch.

Still, others argue that older people really do get worse hangovers, so this one is hard to know for sure.

Hangovers stress you out.

Drinking and being hungover causes a spike in the stress hormone cortisol in your body, which has a long list of effects.

Although we need cortisol to respond to stress, excess levels of the hormone can lead to improper stress responses, altering our mental status, metabolism, and more. Those high irregular cortisol levels can make us less able to deal with the regular stresses of life.

For heavy drinkers, it takes seven days of abstinence for cortisol levels to stabilize.

Binge drinking — the cause of hangovers — at least temporarily wrecks your immune system.

A recent study showed that slamming back four or five vodka shots resulted in serious disruption of people's immune systems both two and five hours after those drinks.

The aforementioned cortisol spike further suppresses our immune systems, diminishing our ability to fight off infections.

Being hungover makes you a terrible driver.

Hopefully, we all know that driving drunk is a terrible idea — but did you know it's best to stay off the road the morning after a bingeing session too?

A recent study found that hungover drivers performed as poorly in a driving simulator as drivers with a blood alcohol content exceeding .05%, the standard international measure for drunk driving (in most of the US, it's .08).

This matches previous research that shows that hungover study participants do as poorly (or worse) on cognitive tests and tests of motor skill, attention, and reaction time as participants with a BAC of .08%.

Some researchers think hangovers might be caused by an immune system response.

Researchers have found that hangovers are often accompanied by high levels of cytokines, molecules that signal the immune system to battle an infection.

We get pain, fatigue, and nausea when our immune system kicks in to fight something off, so it's possible that a hangover is causing the same effects.

If so, an anti-inflammatory drug might help relieve those systems, but many of those drugs, like ibuprofen, can cause problems if taken with a lot of booze. (Don't ever take Tylenol or acetaminophen with alcohol, since that can cause serious liver damage.)

Your brain doesn't work right when you are hungover.

Anecdotally, we know this one to be true, but researchers have gone ahead and confirmed those findings.

Along with dizziness, nausea, and anxiety, being hungover affects your working memory, which is required for holding information in your brain, performing mental tasks, and focusing on anything.

Preliminary findings from some studies by the Alcohol Hangover Research Group show that hungover people experience about a 5-10% working memory loss, and those poor hungover souls make a shocking 30% more errors in certain tasks.

Hangovers cost the economy billions every year.

According to the CDC, excessive drinking cost the US something like $224 billion in 2006, and 72% of that — or $161 billion — was due to the loss of workplace productivity.

Hangovers make you tired, but some hangover symptoms also overlap with the lack of sleep that can be caused by alcohol consumption.

Researchers say that it's hard to isolate the hangover itself from the other effects of drinking too much in real life conditions, especially those caused by lack of sleep.

Heavy alcohol consumption can knock you out, but most people experience disruptions to the second half of their sleep cycle if they've been drinking. This further contributes to daytime sleepiness, which has its own negative effects on cognitive and motor skill performance. If you can stop drinking a few hours before bed, that will probably help both your hangover AND your sleep.

But hangovers also frequently lead to low blood sugar, which can make people moody and sluggish.

There's no proof that any hangover "cure" services work.

Lately we've seen some hangover "cure" services start to appear. But while these services can rehydrate you and perhaps relieve some symptoms, it'll be impossible to cure a hangover until we're actually sure about what causes that veisalgia — the medical term for the pain and suffering that follow a long night out.

Mostly, you'll have to make do with time and rest. Getting an IV into your arm is probably not worth the money.

But food helps with hangover symptoms.

While most "hangover cures" aren't shown to do much of anything (sorry, burnt toast and pickle juice), one thing that does help alleviate hangover symptoms is food, particularly, carbohydrates.

Researchers think that this is because the glucose boost provided by eating those brunchy carbohydrates we love helps restore depleted blood sugar levels.

So please, help yourself to some waffles.

A previous version of this post appeared on March 18, 2016.