(Cover) - EN Showbiz - Simon Cowell is reportedly "bemused" by Britney Spears' behaviour on The X Factor.

The pop songstress was recently named alongside Demi Lovato as a new judge on the US version of the hit talent show.

Music mogul Simon is said to be surprised by Britney's antics during filming.

"Simon's worked with a few big characters in his time, but he's bemused by how demanding Britney can be," a source told British magazine Closer. "Her team have a lot of opinions about everything from her food and wardrobe to who's allowed to talk to her.

"And she's so unpredictable - sometimes she just wanders off because she needs 'some down time.'"

Britney famously had a breakdown in 2008, and there have been some fears over her new high-profile job on the show.

Insiders say Britney can be up and down throughout the day. It has been reported that the star is suffering from Attention Deficit Hyperactivity Disorder, but is unable to take medication for it.

"She can be enthusiastic in the morning but sometimes, she loses focus," a source added. "She doesn't storm off, but she'll just leave because she gets hungry or wants to chill. Everyone gently reminds her there's a schedule, but filming stops a lot." (C) Cover Media