Abstract
This study investigated the relationship between insomnia frequency and objective short sleep, a phenotype of insomnia. The hypothesis is that individuals with objective short sleep simply have more nights of insomnia per week. Data from a randomised controlled trial of Cognitive Behavioural Therapy for Insomnia combined with zolpidem and/or additional maintenance sessions were analysed. Insomnia frequency was derived from pre-treatment sleep diaries, using two definitions: nights with self-reported sleep latency (SL) or wake after sleep onset (WASO) > 30 min, and nights with self-reported sleep duration < 6 h. Objective short sleep was assessed via PSG sleep study, using < 6 h as the threshold. Results showed that objective short sleep was associated with more frequent insomnia when defined as nights with sleep duration < 6 h (8.1 ± 4.2 vs. 5.9 ± 3.7 nights/fortnight for short sleepers vs. normal sleepers, respectively), but not for nights with SL or WASO > 30 min. While insomnia frequency may partially explain objective short sleep, other factors likely influence this phenotype. Further studies are needed to explore the mechanisms underlying objective short sleep insomnia.