STUDY OBJECTIVES: To establish if insomniacs' underestimation of sleep time is due to reduced ability to discriminate between sleeping and waking states. DESIGN: Two night's home polysomnography were compared to sleep diaries. Five laboratory nights employed a series of recorded questions regarding perception of prior sleep-wake state, which were presented during sustained wake and interrupted Stage 2 and REM sleep. SETTING: Sleep laboratory and participants' homes. PARTICIPANTS: Fourteen insomniacs were compared to 8 good sleepers. Mean age for both groups was 58 years. INTERVENTIONS: N/A. MEASUREMENTS AND RESULTS: A signal detection theory analysis was applied to participants' responses to questions presented overnight in the laboratory concerning judgement of prior sleep-wake state and confidence in their decision. Insomniacs had reduced sleep-wake discriminability in addition to a greater bias toward reporting prior wakefulness in the laboratory compared to good sleepers. These measures correlated significantly with the degree of underestimation of total sleep and overestimation of wake recorded at home. CONCLUSIONS: Insomniacs' underestimation of total sleep time is the product of prior sleep being misperceived as wake time upon awakening overnight. This misperception may play a role in the perpetuation of insomnia.
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