Brain temperature in the unanaesthetized chicken: its circadian rhythm of responsiveness to light.

Temperatures were recorded continuously for up to 4 days in the brain of 8 unanaesthetized, moderately restrained male chickens, kept in light-dark cycles (LD) of 30:30 or 60:60 min duration. In all cases, brain temperature was higher in L than in D. Temperatures increased or decreased immediately after the light was turned on or off, respectively, but did not reach a new level until after about 30 min. Average values of temperature obtained during the last 10 min in L and D were used for further analysis. The changes of brain temperature due to changes in light intensity were super imposed to marked circadian oscillations. The computed differences between L- and D-values were found to be a function of the circadian phase. For 4 out of the 8 chickens a uniform circadian rhythm of responsiveness to light could be demonstrated, with two maxima coinciding with the ascending and descending circadian slope, respectively. In view of the evidence suggesting brain temperature as a reliable measure of the degree of arousal, it is concluded that light has a stronger arousing effect on the chicken in the morning and in the evening than in the middle of the day or the night.