One of the first signs that you're not in your early twenties any more is an inability to pull an all-nighter without feeling like you're moments from death the following day. Then you go to an epic party and suddenly it takes so much longer to recover from an equally epic hangover. So what's going on?

Hangovers are fairly simple to explain - it's basically your body trying to filter out an excessive amount of toxins that didn't taste nearly so toxic when you were chugging down your eighth beer the night before. Alcohol irritates the stomach lining, dehydrates you, and depletes blood glucose levels, potentially making you nauseous, tired, sluggish, and irritable.

Our liver processes alcohol by oxidising it with the help of several enzymes, most notably alcohol dehydrogenase. As a byproduct, you get a highly toxic compound called acetaldehyde, which is carcinogenic and causes cell and tissue damage.

Thankfully, acetaldehyde doesn't hang around in the body for too long. It's unstable and quickly gets broken down into a less toxic - but still bad - substance called acetate. However, there's only so much the liver can do to take care of this process efficiently. We can metabolise about one drink per hour, and the alcohol does build up in the body. It's hypothesised that the huge hangover you get after drinking too much is largely due to acetaldehyde flooding your bloodstream.

So why do hangovers hit us so much harder as we age? "Many factors appear to be involved in worsening of hangover in old age. One is that the liver capacity to cope with the toxicity of acetaldehyde decreases as we get old," toxicology researcher Young Chul Kim explains to Melissa Dahl over at NBC News. One of the antioxidants that helps the liver deal with acetaldehyde is called glutathione, and Kim has studied its performance in lab mice. "Our data indicated that, as age increases, glutathione generation capacity is decreased, so cells may not be recovered or repaired rapidly."

The fact that the body slows down as we age isn't really surprising, but there's more to hangovers that just slower liver function. As you get older, you might pack on extra weight, which also changes how you experience the effects of alcohol. "When one's body weight has increased, blood alcohol level decreases because of its wide distribution into body mass and fat, which leads one to drink extra glasses without realising it, subsequently resulting in generation of more acetaldehyde," says Kim.

Losing weight might lead to worse hangovers, too - if you keep drinking the way you did when you were fatter.

In the end, increasingly bad hangovers are simply a reminder that you really are not that young anymore, and maybe should tone down your drinking a bit. Not least because for now, science has not come up with a hangover cure - no matter how many burgers you chow down on at 3am, it probably won't help.