Energy expenditure and physical activity in domestic fowl kept on standard and interrupted lighting patterns.

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.