The half hour you sneak in with a crossword puzzle or Sudoku every morning isn't a waste of time. It's one of your healthiest habits. Here's why:

Neuroscientists at the University of California asked healthy senior citizens to list how often they engaged in mentally challenging activities, like reading, writing letters, playing games, and doing puzzles. Researchers also wanted to know how often the volunteers had done those same activities throughout their life, since they were kids. Then, they gave each senior a brain scan, looking at levels of the proteins doctors believe can cause Alzheimer's.

The result: Seniors who did the least mental activity had brain chemistry that was frighteningly similar to people who'd been diagnosed with Alzheimer's. But the senior citizens who had a long history of challenging their minds had fewer deposits. In fact, the most mentally active seniors had brain scans comparable to people in their 20's. What's this mean to you?

The more hours you spend on mental activities, the better. So, do something challenging every day. And you don't even have to learn Greek.

Study researchers say you might even get away with playing Angry Birds. But before you commit to spending your time launching toucans at green pigs, consider this: Today's seniors weren't playing videogames when they were kids. And the tried-and-true ways of keeping your brain resilient are reading, writing, doing puzzles, and learning new skills.