Slaying medical myths is like playing whack-a-mole: no sooner do you eliminate one than another pops up. Last year Rachel Vreeman and Aaron Carroll of Indiana University School of Medicine exposed seven medical beliefs as myths (more on this below), and now they are refining their aim: in a paper in BMJ, the duo shows that seven medical beliefs related to Christmas are as shaky as an underdone plum pudding:

Sugar makes kids hyperactive? “While sugarplums may dance in children’s heads, visions of holiday sweets terrorise parents with anticipation of hyperactive behaviour,” they write. But no matter what parents believe, sugar has gotten a bum rap. At least a dozen rigorous studies and have compared how children behave on diets containing different levels of sugar, and not a single one found any differences in behavior between the children consuming lots of sweets and kids who never came within hailing distance of a sucrose molecule—not even kids considered “sensitive” to sugar.

What happens instead is that parents who believe their children have gulped down a sugary drink or other goodie rate their children’s behavior as more hyperactive. “The differences in the children’s behaviour were all in the parents’ minds,” the scientists write.

Suicides increase over the holidays? The stress of family, loneliness and more depression during cold, dark months may make more people consider ending it all, but “there is no good scientific evidence to suggest a holiday peak in suicides,” the authors write. One U.S. study of suicides over 35 years found no increase before, during, or after holidays, and people are not more likely to kill themselves during dark winter months: suicides peak in warmer months and are lowest in the winter.

Poinsettias are poisonous? Apparently no amount of reassurance from public health officials that poinsettias are safe seems to get through to some people. But in an analysis of 849,575 cases of people eating plants, none of the 22,793 cases involving poinsettia caused significant poisoning. No one died, and 96 percent did not even require medical treatment—not even the 92 children who chowed down on poinsettia as if it were arugula.

You lose most heat through your head? Even the U.S. Army Field manual says you lose “40 to 45 percent of body heat” through the chimney effect. But as the Indiana scientists point out, “if this were true, humans would be just as cold if they went without trousers as if they went without a hat. But patently this is just not the case.” There is nothing special about the head and heat loss.

A midnight snack—just one more slice of fruitcake!—makes you fat? A study here and there has found that obese people report eating more meals in the afternoon, evening, or night than non-obese people. But just because obesity and after-hours eating go together doesn’t mean the latter causes the former. A midnight calorie counts no more than a noon calorie, which is why studies find no causal connection between eating at night (that is, not eating at night in addition to eating at regular times) and weight gain.

You can cure a hangover with aspirin or bananas or anything else? “No scientific evidence . . . supports any cure or effective prevention for alcohol hangovers,” the authors point out. Randomized trials of various “cures” have come up empty . Prevention, not cure or even treatment, is the only solution, though since alcohol can dehydrate you you may feel better if you drink (booze-free) fluids.

Vreeman and Carroll debunked medical myths unrelated to the holidays in a BMJ paper last year:

• People need to drink at least 8 glasses of water a day? “Complete lack of evidence.”
• We use only 10 percent of our brains? We use much more.
• Hair and fingernails grow after we die? No, though skin and flesh may retract, making it seem that nails are hair are growing.
• Shaving makes hair grow back faster, darker or coarser? “Shaved hair lacks the finer taper seen at the ends of unshaven hair, giving an impression of coarseness. Similarly, the new hair has not yet been lightened by the sun or other chemical exposures, resulting in an appearance that seems darker than existing hair.”
• Eating turkey makes you sleepy? “Any large solid meal (such as turkey, sausages, stuffing, and assorted vegetables followed by Christmas pudding and brandy butter) can induce sleepiness because blood flow and oxygenation to the brain decreases . . . . Accompanying wine may also play a role.”

Or, at this time of year, eggnog.