Recent studies have shown that animals are affected by night-time light exposure. Light is a continuous variable, but our knowledge on how individuals react to different light intensities during the night is limited. We therefore determined the relationship between night light intensity and the behaviour and physiology of great tits (Parus major). We measured daily activity patterns and melatonin levels in 35 males exposed to five different light intensities and found strong, dose-dependent effects. Activity onset was increasingly advanced, and activity offset delayed with higher light intensities. Furthermore, night-time activity increased and melatonin levels measured at midnight decreased with higher intensities. In this experimental study, we demonstrate for the first time dose-dependent effects of artificial light at night on birds' daily activity patterns and melatonin levels. Our results imply that these effects are not limited to a certain threshold, but emerge even when nocturnal light levels are slightly increased. However, in a natural area, these effects may be limited as artificial light levels are commonly low; light intensities drop rapidly with distance from a light source and birds can avoid exposure to light at night. Future studies should thus focus on examining the impact of different intensities of light at night in the wild.