Pre-registration: Examining the Impact of Light Exposure on Emotions, Psychic Health and Behaviours

The selection of participants for the study will be limited to healthy adults (18+) without serious visual impairments. A total of 500 participants are planned to be recruited.

This study will employ an experimental design, a randomised controlled trial (RCT) where participants will be allocated to either the intervention or control group using simple randomisation techniques. Informed consent will be obtained from all participants before participating in the study. Participants will be unaware of the research hypothesis and any details about the light exposures, including those in their own group, until after exposure. Participants will be randomly assigned to either the intervention group (blue light exposure) or the control group (white light exposure) using a simple randomisation procedure, the randomization will also include experiment control conditions such as AB, BA order for the association task. A list of exposures will be ordered in a Latin-square and randomly allocated to participants in blocks giving each participant equal chance of being placed in either group, while ensuring equal group sizes. This method reduces selection bias and ensures that group allocation is unpredictable. By applying simple randomization, the study aims to achieve balanced groups, allowing for a fair comparison between the intervention and control conditions. Light exposure will be provided by two Pracht Luminaire TUBIS LED INDUSTRY fixtures. In the intervention group, a fixture with narrow-band blue light (λ: 469-480nm) will be used, while in the control group, another fixture with broad-spectrum white light (λ: 380-700nm) will be used. Both groups will be in a noise- and temperature-controlled room, isolated from outside interference and natural light sources.

We aim to recruit a diverse sample to represent the general population in Stockholm, including a variety of age ranges and backgrounds, using a self-selection recruitment technique. While achieving this level of diversity is ideal, it may not be attainable due to the nature of volunteer sampling. Recruitment will take place through advertisements on Karolinska Institute’s website and bulletin boards around Stockholm. Participant booking will be done via phone and email, primarily during daytime hours, but evening slots may also be available if it is more convenient for the participants. All collected data are anonymous, and there is no possibility of linking personal data to the collected dataset. The signed informed consent forms are stored without any internal order, which prevents information about the timing of data collection from being used to link personal data to the dataset.

A complete dataset has not yet been collected. The uploaded data material is simulated based on the data collected thus far.

The study will take place at the Department of Psychology, in the Sleep Laboratories at Stockholm University (Albano Campus). The entire duration required for each participant will be a single 60-minute session with no follow-up appointments necessary. First, written informed consent will be obtained from all individuals who volunteer to participate in the study by researchers trained in this process. This includes providing an information sheet containing details about the study’s purpose, what participation involves, potential risks, the type of data to be collected and how it will be stored, compensation, their rights as a participant, as well as contact information for the principal investigator (huvudansvarig forskare). During this time participants will also be given an opportunity to ask questions before proceeding. Those that consent to partake will first undergo baseline assessments. This begins with measuring the resting heart rate of participants using an electrocardiogram (ECG). Participants will place the electrodes on themselves under the guidance of the study coordinator. The electrodes will remain in place throughout most of the data collection process to provide continuous monitoring. Baseline questionnaires will then collect demographic information, as well as measure participants’ current emotional state and stress level (see appendix). Next, maximum hand grip strength will be measured using the participant's non-dominant hand to determine the endurance setting (30% of their maximum grip strength) for the behavioural perseverance task. This ensures that the endurance task is equally challenging for all participants. Lastly, a baseline test will be conducted to assess participants’ initial proficiency in the N-back task before proceeding with the main experimental phase. During the next phase, participants will be allocated to either the intervention or control group using block randomisation. Three tasks will be completed to measure behavioural perseverance (BP) with the order of tasks varying for each participant through Latin square randomisation. The hand grip endurance task will be carried out as follows: Using their pre-determined endurance setting, participants will be instructed to clench the hand grip device and hold that strength for as long as possible. If the participant sustains their grip for five minutes, the study coordinator will end the task. Perseverance is measured by how long the participant maintains the grip. The N-back task will be carried out as follows: The initial baseline assessment will have determined the appropriate starting level of difficulty to ensure that each participant faces an equal challenge. Participants will be instructed to complete the task on a computer and continue until they either choose to quit or reach a predetermined duration (to be established during pilot tasks). Perseverance is measured by how long the participant persists through the task. The thread and needle task will be carried out as follows: Participants will be instructed to simply thread a needle. They will not be informed beforehand that the task is intentionally designed to be frustrating. The width of the thread will make it impossible to complete, especially within the given time limit and under the rules, which state that participants cannot use their mouth or the other hand and must only hold the thread beyond a knot located approximately 5 cm from the tip. Participants will be asked to press a button on the keyboard when they either complete the task or decide to stop. The time taken before pressing the button will be recorded. If the task is not completed within five minutes, an automatic instruction to end will appear on the computer screen. The level of performance will be assessed by how long participants persist in trying to thread the needle. After each task, participants will complete a single-item stress scale and a single-item effort scale. Participants will continue under either the intervention or control lighting condition and then fill out a brief questionnaire assessing their current emotional state, depression, anxiety, stress, suicidal ideation, as well as a hope scale (see appendix). Once these components have been completed, participants will be able to remove the electrodes from themselves as this marks the end of the experimental phase. The post-experimental phase comprises two final components. First, all participants, no longer exposed to experimental lighting conditions but instead normal room lighting, will complete the single-item stress scale one last time and another questionnaire measuring psychological symptoms and transportation habits (see appendix). Lastly, associations to both lighting conditions will be assessed for all participants through free recall of up to five words associated with each condition, along with an estimation of the perceived strength of the association in relation to the light. The order of exposure to the lighting conditions will be randomised using block randomisation, alternating between blue-first and white-first for each participant.

Sample from Stockholm County in Sweden; the home address of participants is not known