Excessive Daytime Sleepiness

The same sample size and inclusion criteria were used in three previous studies from our group [9 , 19 (link), 20 (link)]: 15–19 years of age, no known health conditions, no sleep disorders, body mass index (BMI) of 30 kg/m² or less, not a habitual short sleeper (actigraphically measured TIB <6 h averaged across weekdays and weekends, with weekend sleep extension ≤1 h), consumption of 5 cups of caffeinated beverages or less a day, and no travel across more than two time zones 1 month prior to the experiment.
A total of 126 adolescents were assessed for eligibility for this 15-day parallel-group study. Of these, 60 (30 males) were randomly assigned to the split sleep group (n = 30) and the continuous sleep group (n = 30). Two participants dropped out, and analyses were based on 58 participants (Supplementary Figure S1). Although the primary goal of the current work was to compare two sleep restriction schedules, the data generated were also appraised in light of the recommended sleep duration for adolescents (8–10 hours per night). To this end, we compared the present findings to previously published data on students sleeping 9-hour TIB at night [19 (link)], recruited using the recruitment criteria used in the present study.
The three groups were similar in multiple measures assessed during screening, including age, sex, and BMI percentile (based on the Singaporean BMI-for-age growth charts), as well as daily caffeine consumption, morningness–eveningness preference [21 (link)], excessive daytime sleepiness [22 (link)], and symptoms of chronic sleep reduction [23 (link)] (p > 0.10; Table 1). Although the split and the continuous sleep groups did not differ in sleep behavior based on both self-report [24 (link)] and actigraphy (Table 1), some slight differences were found with the control group from our previous protocol 3 years ago. Specifically, the control group seemed to sleep less on weekdays, but extended their sleep more on weekends. Thus, critically, actigraphically assessed total sleep time (TST) averaged across the week was comparable across all three groups (p > 0.66). Overall, based on actigraphy data, the three groups spent about 6.1–7.0 hours per night in bed on school nights, with more than an hour of sleep extension on weekends. This was far less than the recommended sleep duration of 8–10 hours for adolescents [1 (link), 2 (link)]. Self-reported nap duration, which was not assessed in the control group, averaged 1 hour in both split sleep and the continuous sleep groups (Table 1, p = 0.75).
During the week prior to the experiment, napping was not allowed and a 9-hour nocturnal sleep schedule (23:00–08:00) was enforced for minimizing the effects of prior sleep loss and for facilitating stable circadian entrainment. The split and the continuous sleep groups did not differ in actigraphically assessed TIB (mean ± SEM for continuous sleep: 8.99 ± 0.06 hours vs. split sleep: 9.07 ± 0.08 hours, p = 0.43) or TST (7.37 ± 0.08 hours vs. 7.44 ± 0.10 hours, p = 0.62).