The Associated Press

Among the many mysteries of David Letterman: how much time he's planning to take off this summer.

The CBS Late Show (11:35 p.m. Monday-Friday, Channels 12, 7) host turned over the reins to Tom Arnold last Friday. Letterman's spokesman, Steven Rubenstein, said this week that Letterman will make up his mind whether he's taking more time off as he goes along.

"I've worked since I was 11 years old," Letterman said on his show last week. "And I just feel like it's summer now, I'd like to take a day off."

He told bandleader Paul Shaffer that he had "T.A.S.," or "tired ass syndrome."

The oddity is that Letterman rarely works Fridays anyway. He generally tapes his Thursday and Friday shows back to back on Thursday evening - so, by skipping Friday, he's basically leaving the office a few hours earlier.

Letterman missed five weeks of work earlier this year due to shingles, and was out after a heart bypass operation in 2000.

The time off is notable since he's competing against a workaholic. NBC Tonight show host Jay Leno missed his first night of work in a decade last month due to a job switch stunt with Katie Couric - he was host of Today instead.

Leno has expanded his advantage over Letterman in the ratings over the past few years.

"We were aware he was taking last Friday off," said Chris Ender, CBS entertainment spokesman. "We don't know what the situation is for next Friday. But we don't expect it to be a prolonged situation."

On the cbs.com Web site, the Late Show lists Lauren Ambrose of Six Feet Under and singer Damien Rice as the guests for Friday. There is no mention of a guest host.