Research note: Dark periods modulate activity patterns and alter adrenal gene expression in broilers.

Light-dark cycles regulate circadian rhythms in broilers, yet the physiological effects of manipulating the dark period remain incompletely understood. This study evaluated how different dark period lengths (1, 4, and 8 h) affect activity patterns and stress-related endocrine responses in broilers reared from 14 to 49 days of age. Continuous monitoring of locomotor activity was conducted over 48 hours using wearable accelerometers at 46-48 days of age. The results showed that extending the dark period did not reduce total activity but significantly increased mean hourly activity counts during the light phase (P < 0.01). This indicates that activity was redistributed toward the light phase (n = 6 pens per treatment). Activity was consistently suppressed during the dark phase, suggesting the establishment of a sustained resting state. Plasma corticosterone concentrations tended to decrease in birds exposed to a 4-h dark period compared with those exposed to a 1-h dark period (P = 0.09). In contrast, adrenal expression of genes involved in adrenocorticotropic hormone responsiveness and steroidogenesis was downregulated by extended dark periods. These results indicate that dark periods reorganize the temporal distribution of activity. They also suggest alterations in adrenal responsiveness to stress-related stimulation, although functional adrenal responsiveness was not directly assessed.