The research presented in this paper provides an insight into the behavioural ecology of mountain hares on heather moorland in the Lammermuir Hills of south east Scotland. We examine the seasonal and diel activity patterns using camera traps over a period of 12 months. The rate of camera detections was calculated for the different divisions of the 24h cycle (daylight, dusk, night and dawn). During autumn and winter (October – February), the activity pattern was crepuscular with greater activity at dusk than at dawn. Daylight activity was relatively low and there was a regular pattern of small peaks of activity during the night. In spring and summer (March – September), peaks of crepuscular activity remained evident but daylight activity was much more prevalent than during autumn and winter, and night activity was lower. We discuss the problematic definition of twilight and present an explanation for seasonal changes in the pattern of diel activity that is linked to the reproductive cycle of the mountain hare.