The rotating sleep surface must resist deflection such that there is no interference or contact between it and the lower fixed base throughout its normal range of motion and during operation. This must be maintained with the 9.0 kg test mass placed in all positions within the sleep area, including those that are most onerous (extreme head or toe, side-to-side, and corners of the sleep area). Gaps must remain such that there is no rubbing or interference between parts at any time, and the rollers and center axle assembly are consistently the only points supporting the rotating sleep surface.