1. The energy expenditure (H) and physical activity of laying hens were measured under lighting regimes of 14L:10D (standard), 2L:10D:2L:10D (interrupted) and (15 X (13 min L:47 min D]: 9D (fragmented). 2. Neither of the intermittent regimes produced a significant change in total daily energy expenditure, although large alterations occurred in the distribution of H between the lighting phases. The absence of change in total H resulted from the combined effects of greater H in the light in the intermittent regimes, greater H in darkness during the interrupted day than at night and slightly greater H at night in the intermittent regimes. 3. Physical activity count, like H, was redistributed over time but, unlike H, was also significantly reduced in total; changes in the energy cost of unit activity, however, were such that no reduction occurred in the total amount of H attributable to activity. 4. In the standard groups, 90% of total activity costs were incurred in the light; in the fragmented-day and interrupted-day groups, respectively, 7% and 55% of total activity costs were incurred in the light periods and 85% and 30% in daytime darkness. 5. In terms of both H and activity, there was clear differentiation between night darkness and subjective-day darkness. 6. Even when the energy cost of activity was excluded, significant differences remained between H at night, H in the light and H in daytime darkness.