How is it possible that after the excesses of yesterday's traditional Christmas dinner, some of us could then sit back and absentmindedly munch our way through a tin of sweets or a bumper packet of salty crisps?
For those of you guilty of such a crime (and trust me, it's not just on Christmas Day that it occurs), the jury has returned with a verdict: you're off the hook. Your desire to scoff back a layer of inviting chocolates is in fact a powerful psychological impulse that you have no control over, an age-old phenomenon known as hedonic hunger.
Hedonic hunger has been around for some time -- it's been dressed up in other guises such as emotional eating or stress eating -- and is best described as a nagging tendency to snack on high-salt or high-sugar foods, even when we've just eaten.
Scientists say that hedonic hunger is most prevalent when we think about foods that give us pleasure -- for me, that's a packet of salt-and-vinegar crisps -- and is powerful enough to override our better judgment or lack of hunger.
"Few of us sit around and think about our next serving of broccoli or slice of bread, but we may very well think about snack foods, desserts and foods that have some combination of high-fat, high-sugar, high-salt," Professor Michael Lowe, a psychologist at Drexel University, told the Irish Independent.
"And in those cases we don't need the calories, but we want the simple pleasure of eating those foods."
Scientists have long believed that there are two motivators behind exactly what and how much humans eat. The first, known as "normal" or "homeostatic" hunger, is the regular desire to eat that we feel when our bodies crave food as a means of energy. Those hunger pangs or grumbles in your stomach at lunch time are the normal homeostatic signals between the brain and the digestive system telling you to eat.
But homeostatic hunger's bi-polar cousin is hedonic hunger, a psychological response from the brain's reward centres that has nothing whatsoever to do with your body's need for food. The result, experts say, is that we eat not according to our energy needs but purely for our own pleasure. "We have long known that people are motivated to eat when "traditionally hungry" -- that motive comes from a few different sources but one of them is reward," says Professor Lowe.
"But eating to remove that gnawing feeling, maybe some slight beginnings of weakness and deprivation, is also highly rewarding. And so reward explains eating both when truly hungry and eating when not hungry."
Professor Lowe -- who coined the term hedonic hunger, meaning 'pleasure hunger' -- said that increased neuroimaging technology has allowed scientists to see how the parts of our brain that are dedicated to pleasure "light up" in the presence of palatable foods like those high in salt, sugar and fat.
This response is similar, Lowe says, to the chemical reactions in the brain of an alcoholic or drug addict when faced with the tempting -- and damaging -- cues for their own addiction.
"We know that there is some overlap between the brain areas that are active when people ingest alcohol or drugs and those that are activated when we eat palatable food," Lowe says. "Over time the cues associated with regular drug taking start to produce stronger and stronger feelings of reward and craving. If you translate this to foods where people repeatedly over-consume highly palatable foods that are highly rewarding, then over time the cues associated with the food, more than the food itself, start producing strong motivation to acquire that food."
Hence the reason why when we activate those cues -- sitting on the couch to watch your favourite TV show or curling up in front of the fire with a new book -- the urge to open a packet of crisps or box of chocolates becomes irresistible.
With soaring levels of obesity both in the US and in Ireland, some health experts say that hedonic hunger may be responsible for the mounting levels of obesity.
Weight Watchers UK has publicly decried the prevalence of "food porn" in our society -- the bombardment of consumers with advertisements about high-fat, high-calorie food, assaulting peckish consumers from every angle throughout the day.
The organisation is grateful to the efforts of scientists like Lowe whose work, they say, is helping those clients frustrated by their inability to control their "personal food environment".
"Rather than making us victims of our environment, this emerging science of hedonic hunger has equipped us with the knowledge to better manage weight loss in our overloaded food environment to help our members lose weight with a new simpler focus," says Zoe Hellman, head of public health at Weight Watchers.
The group urges its members to banish high-fat foods such as crisps and biscuits from their immediate reach and instead to have healthy snacks like fresh fruit on hand when they feel the urge to snack.
Professor Lowe agrees, saying that hedonic hunger has always existed in our bodies and that the desire to snack is normal but that our burgeoning calorific consumerist society has made the phenomenon all the harder to resist.
"The desire to have a piece of candy or cake 10 minutes after eating a meal is normal," Lowe says.
"What has changed isn't that set of motives, or the brain that underlies it, but the environment that allows us to satisfy it multiple times a day."