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Five hundred and twelve students aged from 6 to 18 years were administered the morningness-eveningness questionnaire and a sleep log. The mean morningness-eveningness score decreased with grade; this decrease was significant around the seventh grade. The habitual waking time showed no difference among grades but the habitual bedtime became later with grade; thus, the sleep length on weekdays decreased with grade. The percentage of students falling asleep during school lessons significantly increased from the eighth to 12th grade. We found that a delay of sleep phase, reduction of sleep length, increased daytime sleep, and transition to evening type occurred at around the seventh grade.
Shinkoda et al. (Thu,) studied this question.