ATLANTA — Simmons Co., maker of Beautyrest mattresses, held by private-equity firm Thomas H. Lee Partners since 2003, said it plans to file for Chapter 11 bankruptcy protection.

The move will reduce its debt from about $1 billion to $450 million and give it new owners. Simmons, its subsidiaries and parent will be acquired by affiliates of Ares Management and Teachers’ Private Capital, the private-investment branch of the Ontario Teachers’ Pension Plan.