The available literature on sleep systems that include sleep parameters as outcome variables yielded quite contradictory findings. An early study performed in 1957 by Suckling and colleagues, compared objective and subjective sleep
parameters on hard, medium and soft beds [180]. The terms hard, medium and soft were quantified by providing the load-deflection curves of all three supporting surfaces. Four male subjects slept under observation for five nights
ERGONOMICS AND SLEEP: A LITERATURE REVIEW 25 a week during a period of three or six weeks, rotating through three rooms that differed only in the nature of the sleep surface.