Fraying temper and strained relationships over Christmas? Don't blame your in-laws for the heated arguments - you could be suffering from a genuine health problem: low blood sugar.
For years, people have blamed low blood sugar for their irritability or lack of energy (often referring to themselves as 'hypoglycaemic'). But after long dismissing the idea, some experts now believe there may be something in such claims after all.
Furthermore, it's not simply lack of food that causes the problem, as people often assume. It seems that eating the wrong combination of foods - such as a glass of wine and a pudding at lunch, a common indulgence over the festive season - could also lead to low blood sugar.
Whatever the cause, the result is aggressive behaviour, a U.S. study published this year suggests.
A psychologist asked 107 married couples to stick pins into voodoo dolls of their spouses at the end of each day for three weeks. The study, published in the journal Proceedings of the National Academy of Sciences, found that the lower the person's blood glucose levels (measured using an electronic blood glucose meter), the more pins they pushed into their spouse's doll.
In fact, the people with the lowest glucose scores pushed in twice as many pins as those with the highest levels.
Why low blood sugar seems to make people aggressive is still unclear. But it may be because the brain needs 'massive amounts of glucose to run it properly', says Jeremy Nicholson, a professor of biological chemistry at Imperial College London.
How sensitive you are to blood sugar levels depends on individual physiology, adds Professor David Benton, a psychologist at Swansea University who has studied the phenomenon.
'Our work showed that there was a broad continuum among people, from those whose blood sugar levels stayed stable for hours after the standardised meals we gave them, to a minority whose blood sugar levels dropped rapidly soon afterwards,' he says.
'The latter group were the people who reported that they had a tendency to be irritable. It seems they may be physiologically different from the others in some way in how their bodies process glucose.'
The tendency for blood sugar levels to crash also depends on what we eat and drink.
Professor Benton says we are particularly at risk of plunging blood glucose levels if we indulge in the combination of a glass of wine or beer with a sweet dessert or chocolate - this is because, on top of the alcohol's effect, the high sugar content of the sweet treat causes the body to produce a spike of the hormone insulin to mop up the sugar in the blood. Shortly after this, your blood sugar levels may come crashing down - and that's when you get tetchy.
'There is quite a lot of research to show that low levels of blood sugar predispose people to irritability,' he says.
But and there is always a but, as Professor Benton explains: 'If you think you can change a bad relationship into a good one just by giving your partner a biscuit or a piece of fruit to raise their blood sugar, then plainly you are delusional.'