Here's how it happens and what to do about it, according to medical experts.
After her husband passed away, Sandi Bond Chapman said she "could feel it immediately."
"For a year, I couldn't think," said Chapman, founder and chief director of the Center for BrainHealth at the University of Texas at Dallas. "I couldn't write a coherent word. I couldn't do anything. ... Stress is one of the best brain robbers we have."
This isn't merely figurative. According to a recently released Yale University study, stress causes the brain to shrink. So next time you're stressed to the gills and cannot focus, think or remember the ingredients for the meatloaf you make every week, you can legitimately blame stress.
"It's a short, easy story, actually," said neuroscientist and author David Eagleman. "Stress is underpinned by particular hormones that circulate through the body and the brain. Those stress hormones are very bad for brain tissue. They eat away at brain tissue.
"What's new to be stressed about is that stress is literally chewing miniature holes in your brain."
Not all stress poses a problem; our bodies are designed to combat stress by releasing the hormone cortisol. That response grew out of stresses such as, say, being chased by a tiger.
"The general story is that we evolved to have stress systems that are useful when you need a fast response," said Eagleman, director of the Laboratory for Perception and Action at the Baylor College of Medicine in Houston. "What we did not evolve for is chronic stress, that 21st-century stress that man and woman lives with."
Instead of a burst of a stress hormone, most people have chronically elevated levels, he said.
"The body is simply not built to have high levels of stress for long periods," he said. "That's where the stuff eats away at your brain tissue."
The frontal lobe has been identified as "the most critical to everyday functionality," Chapman said. "When you have the impact of stress, things that allow you to be successful will be impaired. You can't figure out how to juggle things, to set priorities."
That's one of four areas of the brain affected by stress. Robert Sapolsky of Stanford University calls the hippocampus "ground zero for stress doing damaging stuff."
"One definitely wants to have a functioning hippocampus," Sapolsky said. "It's all about learning and memory, the part blown out of the water by Alzheimer's disease. It's also the part that is most vulnerable to the effects of stress."
When that happens, "you get neurons dying, shriveling up and losing connections," he said. "It's all really bad news."
The Yale study asked 103 subjects about such everyday stressors as personal relationships and job loss. It found that stress affects areas of the brain that have to do with emotion and self-control, as well as physiological functions such as blood pressure.
"We already know this stuff isn't good for you," Sapolsky said. "If it takes a picture of your brain, seeing pictures of something smaller than it should be, to realize maybe you should make changes, that's great."
He added, "You don't need a picture to realize you're stressed and frazzled."
The definition and effects of stress differ. One person can shrug off what might totally undo another. Generally, stress is considered to come from a feeling of having no control.
"Most people are pretty insightful about whether they're stressed out," Eagleman said. "It's not an all-or-nothing thing. There are varying degrees. For some, their breath is shallow. With others, their muscles ache or their jaw hurts. ... It's important to read your body's signals, to figure out when you're having high levels of stress and structure ways to manage the stress."
When people do, they change, said Dr. Joel Holliner, a psychiatrist.
"When patients come in, we commonly see concentration problems, the inability to make decisions," said Holliner, chief of psychiatry at Medical City Dallas Hospital. "Over time as they get treatment, we see them significantly improve, to going from having memory deficits -- so many think they have dementia or Alzheimer's -- to having their memory totally back to normal. It's fantastic, absolutely."