University students report increased levels of stress in comparison to the general population of the same age, and that may be associated with an increased risk for psychological problems. Depression, anxiety, substance abuse, sleep- and eating problems are often reported by university students. The effectiveness of cognitive, behavioral, and mindfulness interventions focusing on stress prevention among university students was evidenced. However, these programs are generally based on one way to cope with stress. This study is part of a large three-part study and aimed to investigate the impact of a multidimensional stress prevention program on stress, quality of life, psychological and physical indicators, and also psychological resources in university students. The investigators also aimed to study the impact of the intervention on stress and reward experiences in daily life and reward responses under stress in an experimental task.

A randomized controlled trial study was applied to compare pre- and post-outcomes in an experimental group compared to a wait-list control group. Sixty-four university students were randomly distributed in both groups. The experimental group followed an eight-week stress prevention program (GeDStress), integrating cognitive-behavioral strategies with mindfulness-based exercises, emotional regulation strategies, social skills training and assertiveness activities. This program aims at stress reduction and resilience strategies' increase.

The study protocol was accepted by the Ethics Committee of the Cantons of Vaud and Fribourg (Protocol 261/14) and followed the ethical principles of the Declaration of Helsinki (World Medical Association, 2013) and the local regulatory law. Confidentiality was guaranteed and participants could withdraw from the study at any time. All data collected were deidentified with a code. All participants signed the written informed consent.

Before (T1) and after (T2) the participation in the program, both groups participated to a structured interview, which took between 30 and 60 minutes. After that, participants answered well-validated self-reported online questionnaires about their psychological and physical well-being in French. A link with the questionnaires were sent by email and participants had to complete them during the following days. Participants separately received their code number and needed approximately one hour to answer the questionnaires.

At T1 and T2, the students realized also an ambulatory assessment to evaluate their stress and reward experiences in daily life. Participants' stress and reward experience in the daily living environment were assessed five times a day during one week using an adaptation of the Experience Sampling Method (ESM), using an electronic assessment tool. Subjects were asked to report the most important event that happened between the current and the previous measurement and they were then asked to appraise the current situation, their mood, the minor daily events, the related positive and negative affects as well as stress-related experience. In parallel, saliva samples were collected six times a day in order to determine cortisol levels in saliva, to measure the objective level of stress and to combine reported subjective stress with the physiological measures of stress. Saliva samples were obtained by spitting in a tube, and collected saliva samples were stored at -20°C in a deep freeze at the Department of Psychology of the University Fribourg until to be sent to the laboratory for the analysis.

Before and after, the participants also took part in an experimental task to evaluate the reward under stress. The subjects performed the Fribourg reward task developed by Martin-Soelch et al. (2009) to measure reward under a moderate psychological stress. The task comprises two levels of difficulty, which are differentiated through the number of items (3 or 7) to be remembered. The subjects saw first see a fixation cross, followed by the display of the cue about the level of difficulty and whether they can earn money or not during the trial for 1500 ms. After the cue, the subjects saw a fixation cross for 500 ms and then, an array of yellow circles (3 or 7) was displayed for 1500 ms. After a delay, during which a fixation cross was displayed on the screen for 3000 ms, a green circle was presented at any position on the screen for 1500 ms. The subjects had to decide as quickly as possible whether this circle was at the same position as one of the circle presented previously. After the response time has elapsed, the circle disappeared. Half the time, a fixation cross was displayed on the screen for 2000 ms. In the rewarded condition, a feedback about the win and about the cumulated amount of earned money appeared for 1000 ms followed by the balance account for 1000 ms. In the non-rewarded condition, a blank screen was displayed. During the rewarded condition, the subjects could earn a monetary reward. The amount of the monetary reward that could be won varied between two conditions, a low monetary reward consisting in CHF 0.10 or a high monetary reward consisting in CHF 1. The subjects were instructed that they would receive the sum shown at the end of the experiment. The subjects were asked to rate their mood with a visual analog scale in regular intervals. The induction of moderate stress and to evaluate the effects of stress on physiology and brain activation would involve unpredictable mild electric shock induction during the task. At the beginning of each block, the subjects were informed if electric stimulation would be given during the block. In the second block, the electric stimulation can happen at any moment of the trial in an unpredictable fashion for the participant. The intensity of the electric shock was determined before the experiment with each subject individually, to make sure that the intensity of the shock was under the individual pain threshold. In order to measure the level of stress, saliva samples were collected before and after each block (condition), and 10 and 20 minutes after the end of the task, in order to determine cortisol levels in saliva during the different conditions of the task.

Two follow-up sessions were composed by the online self-reported questionnaires, and were measured in both groups at 3 (T3) and 6 (T4) months after the end of the intervention.