People’s health, mood and activities are closely related to their environment and the seasons. Countries at extreme latitudes (e.g. Sweden, UK and Norway) experience huge variations in their light levels, impacting the population’s mental state, well-being and energy levels. Advanced sensing technologies on smartphones enable non-intrusive and longitudinal monitoring of user states. The collected data makes it possible for healthcare professionals and individuals to diagnose and rectify problems caused by seasonality. In this paper, we present a personal mobile sensing system that exploits technologies on smartphones to efficiently and accurately detect the light exposure, mood and activity levels of individuals. We conducted a two year experiment with many users to test the functionality and performance of our system. The results show that we can obtain accurate light exposure estimation by opportunistically measuring light data on smartphones, tracking both personal light exposure and the general seasonal trends. An optional questionnaire also allows insight into the correlation between a user’s mood and energy levels. Consequently, we were able quantitatively inform users with winter blues how little light they were experiencing and also correlate this with their reduced mood and energy, providing evidence for lifestyle changes.