Hummels, 27, still has another season left on his contract but has been linked with several clubs.
Bayern Munich, Barcelona, Manchester United and Liverpool are all reportedly interested.
"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," said Hummels.
Hummels has secured two Bundesliga titles since joining Dortmund permanently from Bayern Munich in 2009 after an initial loan move.
He played in the 2014 World Cup final in Brazil when Germany defeated Argentina 1-0.
His father and agent, Hermann Hummels, says a return to Bayern is not out of the question.
"Mats is facing an enormously important decision in his career, in his life," he added.
"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."
Borussia Dortmund and Germany World Cup-winning defender Mats Hummels says he is losing sleep over his future.