The Gavin & Stacey actor was rushed to hospital last week after falling ill during a performance of Entertaining Mr Sloane in the West End. It was later announced that he was suffering from exhaustion.
Though Horne has already returned to his lead role in the production, friends are said to be "desperately worried" that he has taken on too many projects in recent months.
One source told The Sun: "As soon as we heard what had happened to Mat we all just panicked. We've been fearing that something like this would happen for ages."
The insider added: "To be honest, he was poorly and suffering from exhaustion before he took on the play. Everyone is amazed at how much Mat has been doing. He's always working or out."
As well as working on Entertaining Mr Sloane, Horne has been busy with his BBC Three sketch show Horne & Corden and regular DJing slot at Club XFM in London. He also recently made his film debut in Lesbian Vampire Killers.