Ceiling and floor effects in sleep research

Ceiling and floor effects dictate that the efficacy of sleep-promoting stimuli should be proportional to the degree of pre-stimulus sleep impairment. This review addressed CF effects in polysomnographic research involving hypnotic drugs and exercise. Correlations of placebo/baseline levels of sleep with changes in sleep following hypnotic or exercise treatment were assessed across both literatures. CF effects were further addressed by comparing sleep-promoting effects of hypnotics vs exercise, after ANCOVA control for substantial baseline differences reported in studies of these stimuli. Significant correlations between placebo-baseline levels and sleep changes were observed following both hypnotic and exercise stimuli. Indeed, approximately 60% of the variance in improvement in sleep latency (SOL), wakefulness after sleep onset (WASO) and total sleep time (TST) following hypnotic treatment was associated with differences in baseline levels. ANCOVAs revealed significantly greater decreases in SOL and WASO following hypnotics compared with exercise. However, no significant difference between stimuli was found for TST, and exercise elicited a significantly greater increase in slow wave sleep. Similar results were found when a comparison between hypnotics and exercise was limited to good sleepers. The results show powerful CF influences on sleep responses to hypnotics and exercise and suggest a need for comparing these treatments in poor sleepers.