Mats Hummels has been having difficulty sleeping as he considers his options for next season.

Hummels has been linked to the transfer market for several seasons and that's coming to a possible impasse with his contract expiring with Borussia Dortmund after the 16-17 season.

Bayern Munich, Barcelona, Manchester United and Liverpool have been linked with Hummels.

“It’s a very difficult decision, and when I finally make it people will understand how hard it is for me," said Hummels. "It’s been taking me at least half an hour extra to fall asleep for several weeks now because this is such a big issue for me."

Hummels' father and agent, Hermann Hummels, says he could return to Bayern Munich. Hummels was part of Bayern's reserve team from 2005 until 2008 before his move to Dortmund.

“Mats is facing an enormously important decision in his career, in his life,” said Hermann Hummels. “Dortmund are back among Europe’s best clubs and that is making the decision even harder for him. If he were to leave Dortmund, then he could only go to five or six top clubs, and of course Bayern are one of them.”