Electrophysiological effects of Japanese cedarwood on sleep quality in humans

Sleep plays an important role in human health. Changes in the sleeping environment can affect sleep quality. The present study investigated the effects of natural wood on sleep quality. The interior material was a fully used Cryptomeria japonica wood board (Japanese cedar room), while the particleboard and medium-density fiberboard (MDF) were covered by a vinyl cloth with a woodgrain (resin room). In order to assess sleep quality, the sleep–depth index (SDI) is calculated based on the lengths of sleep stage 3 and the rapid eye movement (REM) stage using electroencephalography (EEG). Before the sleep experiments, the Pittsburgh Sleep Quality Inventory (PSQI) and Morningness–Eveningness Questionnaire (MEQ) were administered to confirm the participants’ usual sleep habits. Thirteen participants were invited to sleep in two different environments. The MEQ before sleep experiments confirmed that eight participants were of the intermediate type and five participants were of the moderate evening type. According to the PSQI, three participants were poor sleepers and ten were good sleepers. The sleep EEG demonstrated that 12 of 13 participants had higher SDI in the Japanese cedarwood room than in the resin room. A Wilcoxon signed-rank test revealed that SDI was significantly prolonged in the Japanese cedarwood room than in the resin room, regardless of the small sample size (n = 13, z = 2.48, p = 0.014, effect size = 0.78). The present study demonstrated that the indoor environment with C. japonica induced a significantly higher SDI compared to that with wood-grain vinyl cloth. This indicates that the interior materials of C. Japonica wood can positively influence the sleep environment.