Chronotype reflects an individual's preferred timing of sleep/daily activities and is typically regarded as a stable trait, though it varies across the lifespan and may fluctuate also over shorter periods of time. Here, we investigate the temporal stability of self-assessed chronotype and its modulation by behavioral advice in a large cohort of University students participating in a 2-arm circadian hygiene education initiative (either encouraging consistency or advancement of sleep, meals, and exercise timing). Students provided demographic and sleep quality information, and repeated chronotype assessments (2-22 months) using the Self-Morningness/Eveningness (Self-ME) question, which offers a choice among 4 categories (definitely morning, morning, evening, definitely evening) and the Ultrashort Munich ChronoType Questionnaire (μMCTQ), which has midsleep and social jetlag as outcomes. Two main samples were analyzed: students completing baseline/2-month follow-up assessments (n = 1902) and students completing baseline/one later assessment, at any time (n = 2820); a subgroup with 3 available assessments (n = 1257) was also examined. Agreement between subsequent Self-ME assessments was good in all samples (0.6 < Cohen's k < 0.8), with approximately 70% of students confirming their first assessment at 2 months, with variations (62%-73%) depending on time between assessments and behavioral advice; transitions mostly occurred between adjacent Self-ME categories. Nonetheless, approximately 30% of participants changed category between 2 assessments, and those who moved once were more likely to move again. "Non movers" exhibited more extreme sleep-wake habits. Furthermore, midsleep and social jetlag changed over the calendar year in ways that seemed to reflect the time-course/constraints of the academic year. The 2 circadian hygiene interventions influenced chronotype stability slightly and differently, particularly in morning types, who probably found it easier to comply with the suggestion of advancing sleep-wake timing. In conclusion, chronotype in young adults shows relative stability but is plastic within limits, also in relation to the way it is measured.
Keywords: assessment; chronotype; midsleep; social jetlag; students.