In the first study of this kind, we systematically assess the effects of mattress firmness on sleep and daytime functioning. Mattress firmness has significant effects on both sleep and daytime function and individuals varied widely in the mattress that optimized their sleep. A convenience sample of 128 healthy adults living in the Raleigh-Durham, N.C. area was recruited to measure the effects of mattress firmness on pain, duration of sleep, sleep quality, and next-day function in “healthy” sleepers. Using a Latin-square randomization design, subjects slept in their homes on 7 test mattresses for approximately 4 weeks per mattress. The firmness of these inner- spring mattresses is typical of mattresses sold in the US marketplace. Mattress firmness was found to significantly affect 9 out of 11 key actigraphic and diary-derived measures of sleep. Though pain was generally minimal for subjects, beds could be distinguished by different levels of morning pain. The “best” and “worst” mattresses were distributed relatively evenly across all mattresses studied.