Could John Mayer be the reason Jessica Simpson was in such high spirits after a late night lunch date in Hollywood on Sunday night?

Jess was snapped looking merry as she left Katsuya restaurant with little sister Ashlee and hair stylist bestie Ken Paves.

There has since been speculation the newly-single star hooked up with ex-boyfriend Mayer while inside the eatery.

“John was in the studio, which is near Katsuya, that night,” a source tells OK!.

“And like he does most nights — like he did again on Sunday night — he went by Katsuya when he took a break from a 12-hour day in the studio.”

Jess leaving Katsuya on Sunday night (WENN.com)

But a more lasting Jess 'n' John reunion is still be unlikely, according to the insider.

The source continues: “He said hello and he hung out a little bit but he left separately

“He’s really good friends with Jess’ brother-in-law Pete Wentz; he sees Ashlee a lot... It’s not out of the ordinary that he sees them and goes over and hangs out for a while.

“They’ve always been friendly. They’re still friendly, but that’s it.”

Jessica Simpson through the years