Background: Sleep disturbance in adults with no health concerns is often linked to the thermal environment. This study assesses the impact on sleep quality of sleepwear made from fibers with different thermal insulation and hygral properties. This randomized cross-over study investigated the effects on sleep quality of sleepwear made from cotton, polyester and Merino wool in adults aged 50-70 years, at an ambient temperature of 30 °C and a relative humidity of 50%.

Methods: Thirty-six healthy participants completed four nights of sleep study with polysomnography. Participants were categorized by body mass index as <25 kg·m^-2 or ≥25 kg·m^-2, age as <65 years or ≥65 years, and by Pittsburgh Sleep Quality Index (PSQI) as poor sleepers (PSQI≥5) or good sleepers (PSQI<5).

Results: Small, but statistically significant sleep benefits were observed for wool over cotton and polyester sleepwear for multiple sleep parameters, while neither cotton nor polyester was responsible for any statistically significant sleep benefit over the 11 sleep parameters examined. The key findings were: 1) A significant sleepwear effect was observed for sleep onset latency (SOL), p=0.04. 2) For older participants, sleeping in wool significantly reduced SOL (12.4 mins) compared with cotton (26.7 mins, p=0.001) or polyester (21.6 mins, p=0.001). 3) A statistically significant effect was found for sleep fragmentation index (p=0.01) in which wool sleepwear (12.1 no·h^{- 1}) was lower than polyester (13.7 no·h^{- 1}) (p=0.005), but not different to cotton (13.3 no·h^{- 1}). 4) Poor sleepers had less wakefulness when sleeping in wool compared to cotton (p=0.047). 5) And Poor sleepers had higher rapid eye movement sleep latency in polyester than in cotton (p=0.037) or in wool (p=0.036).

Conclusion: Statistically significant benefits for wool sleepwear were observed on average for all participants and, in particular, for the older and poorer sleepers. There were no significant differences in any sleep variables between sleepwear types for the BMI sub-group.