## 2 Materials and Methods
### Experimental Procedure  
A dynamic user study was conducted to investigate seating comfort under a wide range of seat configurations with the purpose of sleeping while driving. The main hypothesis of the study was that a more reclined back angle would contribute to occupant comfort and be perceived as a preferred configuration for the sleeping use case in a driving scenario. Subjective comfort and user experience were analyzed to answer the following research questions:  
Figure 1: Prototype seat and environment in the upright, reclined and flat seat positions. (**A**) Upright. (**B**) Reclined. (**C**) Flat. Figure 3: Diagram of the 7-point seatbelt setup on a human body. * Do seat angles affect perceived comfort in real-world conditions? * Does the discomfort of different human body areas and restraint systems affect general comfort ratings? * How does time affect comfort and discomfort ratings? * How do first impressions of different seatback angles reflect their suitability for sleeping? These questions were answered through an experimental evaluation. The study was a moderated in-between designed study with three conditions. The study had repeated measures, and a counter-balanced and randomized design. Participants were welcomed and the instructions were explained. The questionnaire contained 15 questions of varied formats, including multiple-choice, short written responses and fill-in-the-blank answers. The survey was divided into four sections: \"Questions before trial drive,\" which included basic demographic questions; and \"Comfort during the trial drive,\" \"Comfort after each seat position,\" and \"Comfort after the complete trial drive,\" which included comfort questions during and after sitting in the seat, respectively, as well as preference questions. Comfort and discomfort ratings tend to change over time [15]. Therefore, for each condition, the participant rated comfort starting at minute 0, again at minute 10 of the drive and after the drive at minute 15. During the drive, participants were instructed to relax and imagine sleeping during a long-term drive in an autonomous car. Moreover, they were instructed to focus on the seat comfort and their experience. During the drive, questions were orally asked by a moderator, and participants rated the comfort verbally. The moderator transcribed the answers from the subjects. Participants experienced all the three conditions in a randomized order. They had a few minutes between conditions to stand up and take a break before the new condition. The trial drive was conducted in the dynamic track of Volkswagen Proving Ground in Germany over 2 days. The controlled trial drive was designed to be safe and represent close-to-real dynamic conditions. The purpose of the designated drive condition was to represent a constant smooth drive with an autonomous car on a highway. The driving scenario was selected to offer the closest conditions to the use case while maintaining safety standards. The resulting scenario provided a real feeling of driving, including low speed and accelerations, which are suitable for evaluating comfort and trust [16]. Previous studies [17] have evaluated comfort after 15 min of driving motion, as this has been designated as sufficient time for passengers to settle in a moving car [18]. Therefore, the driving scenario involved a 15 min drive per condition at a constant speed of 30 km/h through a dynamic track that included a series of different curves. As safety was a priority, higher speeds and accelerations were excluded due to concerns in the unusual seating position. Moreover, the accelerations were controlled (ay \(\leq\) 0.2 g) during the drive. Each round was approximately 1.25 km and took 5 min. Due to the COVID-19 pandemic, after a robust risk assessment, a special hygiene protocol was established. This procedure, which included measures before and during the test [19], was followed. Before each participant entered the vehicle, the surfaces were disinfected, and the vehicle was aired.