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ObjectivesThe current study examined the relationship between subjective SOL, sleep structure, changes in skin and body temperature, and subjective evaluation in healthy young adults to elucidate the pathophysiological mechanisms of insomnia.MethodsTwenty-eight participants (mean age: 21.5 ± 0.5 years) with no sleep problems participated in a 1-hour polysomnographic recording that obtained objective sleep parameters during the daytime while skin and body temperatures were recorded. The distal–proximal skin temperature gradient (DPG) was calculated. Subjective parameters, such as subjective SOL, sleep time, and restorative sleepiness, were evaluated before and after sleep.ResultsMost participants estimated their sleep latency as being longer than their actual SOL (13.7 min vs. 7.6 min). Objective SOL was significantly correlated with each sleep stage parameter whereas subjective SOL was negatively correlated with stage N2 sleep duration (Rho = −0.454, p = 0.020), slow-wave activity and delta power (Rho = −0.500, p = 0.011, Rho = −0.432, p = 0.031, respectively), and ΔDPG (the degree of reduction of heat loss before and after lights-off). Stepwise regression analysis showed that ΔDPG was the strongest predictive factor in explaining the length of subjective SOL.ConclusionsThe degree of heat dissipation before falling asleep contributed most to the sensation of falling asleep in healthy young adults. This finding may be helpful for elucidating the physiological mechanisms of insomnia and its treatment.STATEMENT OF SIGNIFICANCEThe time estimate ability is also activated during sleep. However, some insomniacs have abnormalities in this function and suffer from sleep state misperception, which is a discrepancy between subjective and objective sleep time. They often overestimate the sleep onset latency. We investigated the relationship between subjective sleep onset latency, sleep structure, EEG frequency components, and body temperature during the process of falling asleep in healthy adults. It was suggested that the Stage N2 duration, the amounts of slow wave activity around after lights-out, and the degree of heat dissipation before falling asleep may be related to the perception of falling asleep. These results are expected to contribute to the understanding of the pathophysiological mechanisms and to the treatment of insomnia.