Piper Jaffray's monthly mattress survey for September, which includes a sampling of mattress retailers across the U.S. representing hundreds of stores, showed slower growth for September, analyst Peter Keith told investors in a research note last night. The survey continues a trend that started with Labor Day weekend and marks a deceleration from the "healthy trend" seen in May through August, Keith writes. He believes "weak" mattress units are the issue, with September showing the largest unit decline in his firm's survey year-to-date. The analyst suspects unit weakness is primarily at the low-end, noting that Tempur-Pedic (TPX) sales "remained healthy." Keith kept Overweight ratings on Tempur-Pedic, Select Comfort (SCSS) and Leggett & Platt (LEG). Mattress unit weakness at the low-end would be most unfavorable to Leggett & Platt, the analyst cautions. Shares of Tempur-Pedic and Select Comfort are both down 5% in early trading while Leggett & Platt is down 1%.