AbstractWhile some activity patterns in seals have been related to tidal rhythms, little work was done on diurnal rhythms until the recent development of image intensifying devices which facilitate night viewing. Individual bulls were studied over several 24‐hour periods and the frequency of behavioural items was recorded during 10‐min periods each hour. No significant differences were recorded between day and night activity levels, except in the frequency of ‘Looks’, nor was there any relationship between activity and tide state. The thesis that optimization of time spent ashore (and therefore ‘at risk’ both ecologically and energetically) is achieved by maintaining a constant level of activity through the 24‐hour period is discussed. It is concluded that night time activity of Grey seal bulls can be inferred from observations made in d