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The actress - who plays troubled sixties suburban housewife Betty Draper in US TV show Mad Men - had been dating the Saturday Night Live star since last July but friends claimed with January working in Los Angeles and Jason in New York, the distance made their relationship unworkable.
A friend told People magazine: "They've been on and off for a few months and then the long distance kind of ended it. But they could get back together. It was just hard to stay together with the distance."
It has previously been revealed that Jason's friends were unimpressed with the 32-year-old blonde, claiming she was different to his usual "smart, funny" type.
A source explained: "His friends don't understand why he's dating her. They call her an airhead behind his back. She doesn't get their jokes. They've only known him to date smart, funny women."
January - who dated Ashton Kutcher from 1998 to 2001 - has previously revealed she wants to date a "real" man.
She said: "What I want is James Brolin in Amityville Horror, minus the horror. A guy with a beard who can swing an axe!
"Dear men of America, I like beer, I like football. I'm probably the most interesting girl you'll ever meet."
