Arguably, no other time of the year has as profound an impact on our behavior as Christmas.
The simultaneous indulgence of sugar, impulse buys and family feuds -- all of which know no boundaries -- can give "It's beginning to look a lot like Christmas," a whole new meaning.
But there might be some biological reasoning behind our seemingly erratic behaviors around this time of year.
Many experts say our feelings, thoughts and actions during the holiday season are driven by hormonal changes that might be more extreme than at any other time of the year.
"Certainly, it brings out the best and worst of us in every which way," said Dr. Robert Lustig, a neuroendocrinologist and professor of pediatrics at the University of California at San Francisco.
Stress might be one of the biggest holiday "S" words, besides "Santa," of course. While the stress factor is different for each person, the biological basis is the same. The stress hormone cortisol works overtime during the holidays, Lustig said.
Cortisol can increase sugar production in the liver to power the muscles, which can increase blood pressure. But stress energy can also turn into visceral fat, which is stored around the waistline. Previous studies suggest that, in general, many people do not gain more pounds during the holidays compared to other times of the year. But the body's composition and even how we perceive our bodies are likely to change.
Stress can also suppress the immune system and bring on colds and flu. And the mixture of stress, temptation and the near-freezing temperatures can get us reaching for comfort food.
Burning the carbohydrates found in the dense, high-energy holiday food can generate body heat and raise insulin levels, Lustig said.
It's great to warm up, but too much insulin can lead to low levels of sugar and can cause the body to crash. Chronically high levels of insulin can lead to diabetes.
The real reason we may reach for that delicious cookie or wrapped package is to ultimately experience that warm and fuzzy feeling of contentment.
Think "Miracle on 34th Street," "A Christmas Story" and "Home Alone." Serotonin, otherwise known as the true happiness hormone, is considered the true meaning of the Christmas season. We can find those elevated levels of serotonin in the final scenes of these generational movie classics. That true contentment is found when the family comes together, when Santa is discovered as the real deal.
Serotonin levels increase only through such feelings as togetherness, not not through material goods such as gifts and food, Lustig said.
Nearly 80 percent of Americans say they feel positive and happy around the holidays, according to the most recent Holiday Stress survey published by the American Psychological Association. Some indicated that the commercial aspects of the season, such as shopping, help conjure those positive emotions.
But because the holiday hype seems to start earlier each year, the stress can last for much longer. So rather than striving for the serotonin high, many opt for the short-lived gratifications that mask serotonin and up the pleasure hormone dopamine.
"When you don't have happiness and when you have stress you opt for simple pleasures, one of which is eating," Lustig said. "The problem is the more you eat, the more your dopamine levels go down, which can make you want to eat more to keep feeling good."
The concept could also translate to shopping off the stress, otherwise known as retail therapy, Lustig said, although studies have yet to prove it.
"Happiness has nothing to do with money. It has nothing to do with food," Lustig said. "The best is to have a community and to be happy with what you've got."
Easier said than done, Mike Ludwig, a professor of neuropsychology at the University of Edinburgh, said in a 2011 editorial in the Journal of Neuroendocrinology. The final feelings of contentment only begin to settle on Christmas Eve or Christmas Day, once the preparation rush, whether we're ready or not, is over.
"The 'love hormone' oxytocin induces feelings of trust and generosity," Ludwig wrote in the editorial.
Nearly 70 percent of Americans feel fatigued by the holiday hustle, according to survey results by the American Psychological Association.
"The experience is sufficiently traumatic that sensible plans are made to do everything different for the next year and to negotiate a multilateral reduction in the extent to which behavior is misaligned," wrote Ludwig.
It might be worth drafting a new game plan sooner rather than later, because the cycle starts again about 300 days after Christmas.
"It is inevitable that, once again," Ludwig wrote, "we will again totally surrender to the effects of our Christmas hormones."