A structured web questionnaire consisting of questions about demographics, health habits, current sleep patterns, history of COVID-19 and COVID-19 vaccination, modified Insomnia Severity Index (ISI), and Epworth Sleepiness Scale (ESS) was utilized.
All participants were asked if they had ever had COVID-19 and whether they had been vaccinated. Participants who received two or more doses of the COVID-19 vaccine were asked if they had any adverse reactions to the vaccine. We used the following questions: “Have you ever had COVID-19?”, “Have you received two or more doses of the COVID-19 vaccine?”, and “Have you experienced any side effects from the COVID-19 vaccination?” (Figure 1 ).
We investigated usual sleep times on workdays and free days. The average sleep time was calculated as follows: (weekday sleep time × 5 + weekend sleep time × 2)/7. Sleep efficiency was defined as the ratio of average sleep duration to average time in bed.
To measure chronotype, we used mid-sleep on free days corrected for sleep debt on workdays (MSFsc), using the formula: MSFsc = midsleep point on free days − (sleep duration on free days − average weekly sleep duration)/2. A higher MSFsc value reflects a stronger eveningness tendency. Based on the MSFsc distribution of their sample, with 2.5% at each end of the distribution as the extreme chronotypes, Kühnle et al. [17 ] suggested that an MSFsc value less than 2.17 should be defined as extreme morningness and that an MSFsc value of 7.25 or greater should be defined as extreme eveningness. We used the midsleep point on free days (MSF) and the midsleep point on workdays (MSW) to quantify social jet lag using the formula: Social jetlag = |MSF − MSW|. The definition and measurement refer to the theory and process presented by Wittmann et al. [18 (link)].
We used the Korean version of the ISI to investigate insomnia symptoms [19 (link)]. The optimal cut-off score of the ISI was 15.5, and the sensitivity and specificity at that score were 0.92 and 0.82, respectively [19 (link)]. The reliability was confirmed by Cronbach’s alpha of 0.92, and the item-to-total-score correlations (item–total correlations) ranged from 0.65 to 0.84 [19 (link)]. The ISI is a brief screening questionnaire that measures insomnia severity. The total score ranges from 0 to 28, with a higher score indicating greater insomnia severity. If a participant’s ISI score was 15 or more, they were classified as having moderate to severe insomnia. In this study, difficulty initiating sleep, difficulty maintaining sleep, and waking up too early were evaluated using the criteria that these issues were experienced five or more times per week.
The Korean version of the ESS is a reliable and valid instrument for screening patients with daytime sleepiness [20 (link)]. The ESS consists of eight sleep-related situations, and participants were asked to assess the likelihood of falling asleep in each situation. The total score ranges from 0 to 24, with a higher score indicating a high level of daytime sleepiness. An ESS score of 11 or more was classified as excessive daytime sleepiness.
All participants were asked if they had ever had COVID-19 and whether they had been vaccinated. Participants who received two or more doses of the COVID-19 vaccine were asked if they had any adverse reactions to the vaccine. We used the following questions: “Have you ever had COVID-19?”, “Have you received two or more doses of the COVID-19 vaccine?”, and “Have you experienced any side effects from the COVID-19 vaccination?” (
We investigated usual sleep times on workdays and free days. The average sleep time was calculated as follows: (weekday sleep time × 5 + weekend sleep time × 2)/7. Sleep efficiency was defined as the ratio of average sleep duration to average time in bed.
To measure chronotype, we used mid-sleep on free days corrected for sleep debt on workdays (MSFsc), using the formula: MSFsc = midsleep point on free days − (sleep duration on free days − average weekly sleep duration)/2. A higher MSFsc value reflects a stronger eveningness tendency. Based on the MSFsc distribution of their sample, with 2.5% at each end of the distribution as the extreme chronotypes, Kühnle et al. [17 ] suggested that an MSFsc value less than 2.17 should be defined as extreme morningness and that an MSFsc value of 7.25 or greater should be defined as extreme eveningness. We used the midsleep point on free days (MSF) and the midsleep point on workdays (MSW) to quantify social jet lag using the formula: Social jetlag = |MSF − MSW|. The definition and measurement refer to the theory and process presented by Wittmann et al. [18 (link)].
We used the Korean version of the ISI to investigate insomnia symptoms [19 (link)]. The optimal cut-off score of the ISI was 15.5, and the sensitivity and specificity at that score were 0.92 and 0.82, respectively [19 (link)]. The reliability was confirmed by Cronbach’s alpha of 0.92, and the item-to-total-score correlations (item–total correlations) ranged from 0.65 to 0.84 [19 (link)]. The ISI is a brief screening questionnaire that measures insomnia severity. The total score ranges from 0 to 28, with a higher score indicating greater insomnia severity. If a participant’s ISI score was 15 or more, they were classified as having moderate to severe insomnia. In this study, difficulty initiating sleep, difficulty maintaining sleep, and waking up too early were evaluated using the criteria that these issues were experienced five or more times per week.
The Korean version of the ESS is a reliable and valid instrument for screening patients with daytime sleepiness [20 (link)]. The ESS consists of eight sleep-related situations, and participants were asked to assess the likelihood of falling asleep in each situation. The total score ranges from 0 to 24, with a higher score indicating a high level of daytime sleepiness. An ESS score of 11 or more was classified as excessive daytime sleepiness.
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