We systematically assessed the effects of mattress firmness on sleep, pain, and daytime functioning, finding that mattress firmness has statistically significant effects on both sleep and daytime functioning and that individuals varied widely in the mattress that optimized their sleep. A convenience sample of 128 healthy adults living in the Raleigh-Durham, North Carolina, area was recruited to assess the effects of mattress firmness on pain, duration of sleep, sleep quality, and next-day function in healthy sleepers. Our study used a Latin-square randomization design, and subjects slept in their homes on seven 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.