A naturalistic sample of 150 students was recruited across academic disciplines at two public Universities in Regensburg via bulletins, flyers, and personal appeal at lectures over two semesters. Past or present internal, neurological, hormonal, or psychiatric disorders were recorded. Data for the present study were collected in parallel to another survey – for further details regarding sample selection, study design, and time-course see: Zunhammer et al (2013) [2] .

Procedure

Written informed consent, medical history, and exam dates were obtained in a structured interview at a study inclusion visit. Afterwards, data were obtained using an online platform. Participants were asked to complete online-questionnaires before (Pre-Baseline), during (Exam Period), and after (Post-Baseline) an academic exam period. The Pre- and Post-Baseline time-points were scheduled within exam free periods, i.e. at a time-point where no exam was scheduled within the previous and next 30 days. The Exam Period time-point was scheduled within three days of the most “fearsome and/or distressing exam” of the current semester, as designated by the participant at study inclusion. The exam in question had to be prerequisite for graduation and/or contribute to the university degree. Participants were required to report any re-scheduling of exams. Exam dates were re-checked via the online platform at post-baseline. At each time-point, participants received a reminder by e-mail, or, if necessary, by telephone.

Participants were asked to complete an online version of the German PSQI [13]. The PSQI is an established questionnaire for the measurement of sleep quality [14], [15]. It consists of 9 self-report items and an additional item asking for observations made by room/bedmates. The additional item was not assessed in the present study. Higher PSQI-scores denote decreased sleep quality. Poor sleep has been defined as a PSQI global score of > = 5 [13]. This cutoff has been shown to have high specificity and sensitivity for distinguishing insomnia patients and healthy controls [13], [15]. Instructions and wording of the PSQI online form were identical to the original paper version, but missing items were precluded by forced-choice settings. The time frame of assessment was the past month, as in the original version [13].

The Regensburg Insomnia Scale (RIS) is a short rating scale for the assessment of psychophysiological insomnia [17]. It consists of 10 items querying typical insomnia symptoms. Higher scores denote increased symptom severity. Symptom severities are rated on five-point Likert scales. The RIS was included as an additional measure of sleep quality in the present study with a focus on insomnia. The time frame of assessment of the RIS was the past month.

The amounts of alcohol (in drinks), caffeine (in units), and nicotine (in cigarettes) consumed within the past week were surveyed in a separate online form. An alcoholic drink was defined as 150 ml of wine, 333 ml of beer, or 40 ml of liquor (∼ 13 g of ethanol). An unit of caffeine was defined as 200 ml of coffee or tea, or 500 ml of caffeinated soft drinks (∼ 50 to 100 g of caffeine). Again, missing items were precluded by forced-choice settings.

For the present and the parallel study [2], acute infections, injuries, and exacerbations of pre-existing conditions were surveyed using an online form asking for health status and a description of current symptoms. The obtained reports of current health status were categorized into “ill/injured” or “healthy” by three investigators independently. In case of doubt participants were classified as “ill/injured”.

The Perceived Stress Questionnaire (PSQ-20) [16] was obtained as a self-report measure of stress for both the present and the parallel study [2]. The PSQ-20 consists of 20 negatively and positively worded items, asking for typical indicators of stress, e.g. “Your problems seem to be piling up”, “You have trouble relaxing”, or “You have enough time for yourself”. Items are rated on a 4-point likert scale (1 “almost never”, 2 “somtimes”, 3 “often”, 4 “usually”). The time frame of assessment chosen for the PSQ-20 was the past week. The PSQ-20 total score was used as an additional predictor of sleep quality and used to verify that the exam period was perceived as stressful.