Epidemiological Evidence for the Bimodal Chronotype Using the Composite Scale of Morningness

A previously published paper by Martynhak et al. (2010) drew attention to a possible fourth chronotype, the bimodal type, in addition to the traditional classifications of morning types, evening types, and neither/intermediate types. Morning types get up early, are most active in the morning hours, and go to bed early, whereas evening types get up late in the day and achieve their physical and mental peak performance during the afternoon and evening, and go to bed late at night (see, e.g., Randler, 2008a). Previous research has found most people are neither or intermediate types. The Morningness-Eveningness Questionnaire, or MEQ (Horne & Östberg, 1976), was used by Martynhak et al. (2010) and the Composite Scale of Morningness, or CSM (Smith et al., 1989), was used by our group. Both scales contain some items in common and correlate well (∼.9) with one another, but the CSM is assumed to be easier to apply, and it might be easier to understand (see discussion in Smith et al., [1989] and Brown [1993] concerning the BALM [Basic Language Morningness Scale]). In theMEQ and CSM, individuals with a relatively early preference attain more points on the 19 MEQ or 13 CSM questions, which results in a higher total score indicative of morning type, whereas evening types achieve a lower score. The intermediate or neither type should choose the options with an intermediate scoring. However, as Martynhak et al. (2010) pointed out, a small group of individuals has a different pattern of answers. They answer some questions as morning types, getting higher scores for these questions, whereas they answer others as evening types, getting lower scores. As all of the classification items of the CSM are based on some kind of a Likert-type rating, neither types can, broadly speaking, exhibit two different patterns: (1) they often use the values centered around the mean, e.g., values 2 and 3 on a 4-point Likert scale, or (2) they use the “bimodal” pattern of crossing out extreme answers but in a bimodal fashion, e.g., values 1 and 4. They sometimes answer as a morning type and sometimes as an evening type, which in sum will provide the same result of scoring, and when consistently done it will place a person in the neither-type category. Therefore, Martynhak et al. (2010) introduced the new aspect of bimodality to distinguish and discern bimodal types within the neither/intermediate chronotype. Bimodal types may have different activity patterns with respect to the time of day. It is possible that there exists M and E oscillators in mammals, including humans, with two bouts of activity, one in the morning and another in the evening, as has been observed, for example, in primates (for more details, see Martynhak et al., 2010). Here, we put forward the idea of Martynhak et al. (2010) by analyzing a large sample of subjects comprising pupils, students, and workers. The sample of persons was recruited from more than 10 different studies conducted between the years 2005 and 2010. Permission for their study was granted by either a local or university committee, and all study procedures followed recommended ethical regulations, such as outlined in Portaluppi et al. (2010). We analyzed data from 10 123 persons living in Germany and calculated the bimodality index for all items of the total scale, using chi-square statistics to check for the distribution of chronotypes across sex and occupation (pupils, students, workers). To classify chronotypes, we first used the bimodality index (>0) and the 26/43 cutoff scores proposed by Randler (2008a) to distinguish morning from evening types. All other persons were classified as neither/intermediate types. The cross-tab function was used with standardized residuals to assess which specific chronotype × sex and chronotype × group deviated from the expected frequencies. The chi-square statistics procedure is based on a crosstab calculation and provides only one overall chi-square