Circadian rhythms of chicken brain temperatures.

1. Brain temperature was recorded continuously for up to 18 days in unanaesthetized adult male chickens. With the use of a guide box of plexiglas screwed into a trephine of the calvarium, several thermocouples could be inserted at various depths into the brain at the same time.2. While brain temperatures were being recorded, each chicken was placed in a small circular arena and kept either in a light-dark cycle (LD 12:12 hr) or in conditions of constant dim illumination (LL) within a soundproof chamber.3. Under LD-conditions, the range of oscillation (the difference between maximum and minimum within one period) in brain temperature at any one site was about 1.5 degrees C. During the 12 hr of light the temperature often reached a plateau for several hours. During darkness, a minimum of temperature was usually reached shortly after light-off. Brain temperature started to rise several hours before light-on.4. All eleven chickens tested under LL-conditions showed free running circadian rhythms of brain temperature, with mean periods varying between 22.75 and 25.00 hr (overall mean: 23.69 hr). The range of oscillation in LL-conditions was smaller than in LD-conditions, but was seldom less than 1.0 degrees C.5. In LD as well as in LL, continuous fluctuations of temperature with a much higher frequency were superimposed on the circadian cycle. The fluctuations occurred synchronously at all sites of the brain and were of the same order of magnitude (frequency and range) during wakefulness as during sleep.