Nearly half of all U.S. workers leave their PCs on overnight. Here, three urgent reasons to turn off before you head out:

1. Hackers spend their waking hours prowling for vulnerable computers. Don't assume your firm has up-to-date security—a third of small businesses don't.

2. Your computer is a carbon-spewing beast. Computers left on standby will emit 20 million tons of carbon dioxide this year—roughly the same amount as 4 million cars.

3. Powering down may save your job. U.S. companies waste $2.8 billion annually keeping computers on after hours—in tough times, that's a heck of a lot of salaries.

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