Participants will record their feelings, emotions and experiences using a smartphone app several times a day. Participants may also be invited to use a wearable device (like a Garmin fitness watch) that, along with data from the smartphone app, will provide information about things like a person’s physical responses (blood pressure and heart rate), sleep, and social context. From this information, we will be able to work out if changes might be a sign of someone becoming unwell. For example, someone who is feeling very anxious or low might not feel like socialising much or might have changes in sleep and physical activity patterns.