Borussia Dortmund captain Mats Hummels has admitted he is losing sleep over whether or not to leave the German club this summer. The 27-year-old star has been persistently linked with a move away from the Westfalenstadion over the last few years, and Hummels has set himself a deadline to decide his future.

The World Cup-winning centre-half has refused to reveal when he will make the crucial decision by, but has admitted it has been causing him sleepless nights. "I've set a date [for a decision], but I'm keeping it to myself," Hummels told Sky Deutschland. "It's costing me half an hour before bed every night."

Hummels has been strongly linked with a transfer to Manchester United in recent years, with the Old Trafford club thought to be keen to acquire a top-class centre-half following the departures of Rio Ferdinand and Nemanja Vidic.

The defender's father, Hermann Hummels, has confessed his son faces a tough decision and has even admitted Mats could make a return to Bayern Munich, where he started his club career.

"Mats is facing a tremendously important decision in his career, in his life," Hermann told Bild of the reported United target. "Dortmund is back among the top clubs in Europe, so the decision is so hard for him. If he should leave Dortmund, then he goes to one of the five or six top clubs. And to these clubs one is, of course, FC Bayern."

Hummels has previously admitted he is considering a transfer away from Dortmund. "Now, if I do sign a long-term contract [with Dortmund], I will be 31- or 32-years-old at the end of the season. At the moment I do not plan to play until I'm 38," he said amid United's supposed interest.

"I want to play a certain kind of football but I also want to win titles. The better the team, the greater the stimulus, to be part of it."