Maartje Boer

CHAPTER 2 34 Sleep Foundation (Hirshkowitz et al., 2015). For 12- and 13-year-olds, at least nine hours of sleep is recommended, whereas for 14- until 16-year-olds, at least eight hours of sleep is recommended. In order to study the association between problematic SMU and low sleep duration specifically, we created a dichotomous variable lower sleep duration than recommended , with categories (1) not meeting the recommendation and (0) meeting the recommendation . Also sleep quality was measured using five items from the Groningen Sleep Quality Scale (Meijman et al., 2006). Adolescents were asked to evaluate their sleep during the past week on schooldays, for example “I felt like I slept poorly last night”. Responses ranged from (1) never to (5) (almost) always , and therefore high values indicated lower sleep quality. The test scores of the five items yielded a Cronbach’s alpha from 0.769. The mean of the five items was dichotomized into the variable low sleep quality with categories (1) mean score above 3.5 and (0) mean score below 3.5 . Demographic Characteristics Gender consisted of two categories: (1) girl and (0) boy . The Dutch education system distinguishes broadly three paths of secondary education: pre- vocational education (‘VMBO’), general secondary education (‘HAVO’), or pre-university education (‘VWO’). Students typically follow one of the three paths. Hence, educational level consisted of categories (1) low (pre-vocational education, i.e, all ‘VMBO’ levels or ‘VMBO/HAVO’), (2) medium (general higher education, i.e., ‘HAVO’ or ‘HAVO/VWO’), and (3) high (pre-university education, i.e., ‘VWO’). Age varied from 12- to 16-year-old. Ethnic background was determined by adolescents’ responses to the question where their parents were born, and consisted of three categories: native (both parents born in the Netherlands), non-Western (at least one parent from Africa, Latin-America, Asia (excluding Indonesia and Japan) or Turkey), and other Western (at least one parent from Europe (excluding Turkey), North-America, Oceania, Indonesia, or Japan, and no parent from a non-Western country) (Central Bureau for Statistics, 2019b).