Houston-based Mattress Firm Holding Corp. (NASDAQ: MFRM) announced that it will acquire HMK Mattress Holdings LLC in a deal valued at $780 million. HMK Mattress Holdings LLC is the holding company of New York-based Sleepy's, which has more than 1,050 stores in the northeast and on the East Coast. The deal is expected to close in either the first or second quarter of 2016, according to a Mattress Firm release. "The acquisition of Sleepy's is an important next step in building our national store…