UP ALL NIGHT” star CHRISTINA APPLEGATE is so stressed out that friends fear she’s headed for a meltdown!

The 40-year-old sitcom sweetheart is struggling to juggle the demands of her weekly series, her fiance, and their 1 1/2-year-old daughter Sadie, sources say.

After battling breast cancer and undergoing a double mastectomy in 2008, “Christina welcomed the distraction of work after she got the offer to star in ‘Up All Night,’” said a source.

“But now she feels that she made a colossal mistake. Most days she’s out the door at 5:30 a.m., and by the time she gets home, she’s exhausted. She’s at her breaking point.”

Her NBC sitcom has been renewed for a second season, but Christina – who shot to fame on “Married…with Children” and a few years ago starred in “Samantha Who?” – hopes to shift gears career-wise, said the source.

“Christina wants to do feature films where she can work intermittently and then go back to a normal life,” explained the source.

“She’ll star in the ‘Anchorman’ sequel with Will Ferrell early next year, and she’s hoping it’s a springboard to making movies.”

Meanwhile, Christina was set to marry musician Martyn Lenoble shortly after their engagement on Valentine’s Day 2010.

“But she just can’t find the time to plan a wedding, and Martyn is getting frustrated,” said the source.

“Christina also realizes her decision to race back to work may have cost her the opportunity to get pregnant again. She desperately needs time off, but she can’t seem to jump off the treadmill.

“She’s falling apart!”