It's marathon season for multitaskers.

With Thanksgiving and Black Friday behind them, now comes the final push: Christmas.

Thirty-one short days to get it bought, baked, cleaned, crafted, catered, whipped, chilled, polished, ironed, ordered, wrapped, stamped, mailed, framed, hemmed, duct-taped and carted to the basement.

Much of it simultaneously.

Via cell phone, e-mail, fax and text messaging as they drive, work, eat, talk, cook, brush their teeth and practice Pilates.

All the while getting dumber.

At least that's what new studies of multitasking seem to show. Mom was right - you can't watch TV and do homework at the same time.

When you force your brain to leap from task to task, you spend so much energy on the mental juggling that nothing of substance ever sinks in.

"This is the great irony of multitasking - that its overall goal, getting more done in less time, turns out to be chimerical," writes Walter Kirn in November's Atlantic Monthly. "In reality, multitasking slows our thinking. It forces us to chop competing tasks into pieces, set them in different piles, then hunt for the pile we are interested in, pick up its pieces, review the rules for putting the pieces back together, and then attempt to do so, often quite awkwardly."

In a widely reported experiment at UCLA, scientists asked students to sort index cards. One time they sorted them in silence, with no distractions. The second time, the students sorted while listening to a series of high and low beeps, keeping track of the high beeps.

Both times, the students sorted the cards equally well. But when they had to listen to the beeps, they did a much poorer job of recalling what they'd sorted.

When you multitask, you may also give your brain extra doses of cortisol and adrenaline, stress-related hormones that erode your hippocampus, where memory is stored.

For my generation, which one could argue pioneered the dark art of multitasking, the part about your brain being eaten is unsettling. For us lifelong jugglers, it's a body blow, akin to how the pious must have felt to learn that Mother Teresa doubted God's existence.

For years, we thought doing 18 things at once was enlightened. Multitasking, in fact, was a badge of honor. If a woman wasn't barking orders to her staff on her cell phone, driving, making a grocery list for her dinner party and disciplining her kids in the back seat, she wasn't worthy of her gender.

And she passed it on to her kids, who, thanks to technology, have become so overstimulated and overextended that they can't do homework without an Ipod in one ear and a cell phone in the other while also updating their pages on Facebook with their laptops, in front of the TV.

If you think that's hyperbole, think again.

A 2005 Kaiser Family Foundation study of middle- and high-schoolers found that 58 percent multitask while reading and 62 percent do so while using the computer.

The problem is the same at the workplace. Companies are taking small steps, like declaring e-mail-free Fridays, because employees A) get so many e-mails they quit reading them, even the important ones, and B) forget the value of interacting face to face with people.

The findings about multitasking have caused a sea change in my thinking. Do I regret spending 20 years of my life juggling family and career at 100 mph?

Yes, I do. At least what I can remember of them.