To cut costs, Yahoo plans to close its U.S. offices for a week at year's end, according to an email sent to employees.

For the first time in Yahoo's 11-year history, its 10,500 U.S. workers will be told they must take their vacation time at the end of the year if they want to be paid for the time off, reports the Associated Press. Yahoo would lower lighting and heating expenses and reduce liabilities for employee accrued vacation time.

Coming after warnings that revenues will fall a bit below expectations because of online ad growth is plateauing, the move caused some anxiety among investors. Yahoo's stock price has fallen 35 percent so far this year.

Silicon Valley gossip site Valleywag.com last week published the Yahoo email, in which Yahoo SVP for human resources Libby Sartain wrote, "Taking a little time off during a work week when so many of our partners and advertisers are also closed is the prudent thing for Yahoo to do."

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