Light-immersion data from geolocators can be used to estimate behavioural budgets and energetics in seabirds throughout the annual cycle. However, all methods used to categorise time spent in behaviours rely on assumptions that are difficult to validate. Additional data, such as pressure and temperature data from time-depth recorders (TDRs), can help to refine these assumptions. We explore the utility of previous methods to derive behavioural budgets from light-immersion data using a dataset from Common Guillemots Uria aalge, where individuals were equipped with both a TDR and a solar Global Location Sensor (GLS), also known as a ‘geolocator’. We compared behavioural allocations from previous methods to those derived when also using TDR data. Previous methods used light-immersion data to distinguish between time foraging, active, and resting on the water, but the addition of TDR data revealed that these activities resulted in similar light and immersion levels. It was also more difficult to differentiate between rest and flight using light-immersion data alone. However, by using insights gained from combined light-immersion and TDR data, we developed an improve dmethod to assign behaviours using light-immersion data alone, and provide an adjusted equation to use these data to calculate energetics in Guillemots. We recommend using our approach when processing light-immersion data; however, if detailed activity budgets (particularly foraging information) are required, we recommend using higher resolution loggers, e.g. integrated light-immersion-temperature-pressure devices. Our findings are likely to be relevant for studies of other seabird species (particularly other auks) that dive and spend most of their time at sea during winter.