It washes over you roughly half an hour after you finally put down the cutlery at Christmas lunch.

The wave of tiredness that makes your eyelids droop and brain fade, right when you're meant to be entertaining friends and family.

It's the postprandial dip, and if it's an extreme one you might call it a "food coma".

If it happens after lunch at work, an afternoon guarana all natural energy shot or stroll around the block should be enough to get you back up and running.

But if you've had an abnormally large meal — such as a festive season feast — it may feel like you have no choice but to let the food coma take hold.

So, what do we really know about this holiday hazard, and is there anything you can do if you find yourself heading towards post-food torpor?

Let's take a look at some of theories on why eating can be followed by feelings of fatigue.

Rest and digest

The science of food comas is mostly based on animal models and inferences from our current knowledge on how the body works, says Emma Beckett, a nutrition scientist at the University of Newcastle.

There are three main theories, and Dr Beckett thinks they probably all play a role to some extent.

The first theory involves the activation of the parasympathetic nervous system — that's the rest and digest pathway, as opposed to the sympathetic nervous system which is fight or flight.

"We've got a bunch of nerves around the digestive tract. And one of those big nerves — the vagus nerve — signals between the brain and the gastrointestinal tract," Dr Beckett says.

"From an evolutionary point of view, if you've come across abundant food — the feast period — you don't want to be going running around and risking losing that food."

And Dr Becket says there's no reason why you shouldn't take a timeout on the couch, if that's what you feel like doing.

Blood flow and insulin levels

There is a myth that we feel drowsy after eating because the blood is directed away from our brain to our gut, but that has been disproven, according to Dr Beckett.

"But there is evidence that your body prioritises getting blood to your gut over your muscles, which could make you feel a little bit weak," she says.

So theory two is that while your body directs some blood flow to your gut to aid digestion, the blood flow to your muscles might decrease slightly and leave you feeling a bit lethargic.

The third theory is all about the role of insulin in your body.

Insulin is a hormone that allows us to use sugars from the food we eat for energy, and it might also help amino acids get to the brain, says Dr Beckett.

"Some of those amino acids are used to make your relaxation hormones or are part of the relaxation signaling pathway," she says.

"So when we've got high levels of insulin, it might be easier for amino acids like tryptophan to get into the brain."

Tryptophan is an essential amino acid used by the body to make serotonin — and thus can make you feel sleepy.

More insulin can also increase the activity of the energy pathways in our cells — the moving of things, like potassium, out of cells and into the other fluids.

"This can also make you feel a little bit of fatigue or muscle weakness, because that's normally balanced further inside of the cells," Dr Becket says.

"So all of those things are theories about how it works, and it's probably a little bit of all of them."

Basically your body is busy processing your food, so you need to relax and divert energy while that happens.

More food, more food coma

High carb food that's going to increase your insulin levels is more likely to contribute to a food coma, whereas a balanced meal with less fat and carbs and more protein will have less of an effect.

And logically, the greater the amount of food you eat, the more each of those theories is going to come into play.

More carbs means more insulin.

More food in your gut means more parasympathetic nerve activation.

And more food to digest means more blood flow that's going to be needed to help.

So the obvious way to avoid a food coma these holidays is to eat less.

A food coma itself won't hurt you, but it is an indication that you've probably eaten a bit too much, which, again won't hurt you if it's a one off.

"If someone was wanting to go and have a nap after lunch every single day, then I'd probably have questions about how much and what they were eating for lunch," Dr Beckett says.

"But you're not going to increase your risk of disease based on one meal that happens once a year.

"People can just relax and enjoy themselves as long as you are eating a healthy, balanced diet for the rest of the year.

But if the soporific power of the holiday feast becomes too much for you, going for a walk might help that food coma feeling pass a bit faster.