The Gavin & Stacey star collapsed on stage last night during a performance of Entertaining Mr Sloane in London.
He lay on the floor for ten minutes before paramedics arrived and took him to hospital, but he was well enough to leave that evening, The Guardian reports.
Publicist Rupert Fowler said: "Mat has been working flat-out for the last three months. He has patently been suffering from exhaustion."
Horne was replaced by understudy Fergus March for that evening's performance.
No decision has yet been made on when the actor might return to the play, which is due to close on April 11.