Energy intake and the circadian rhythm of core body temperature in sheep

We tested the hypothesis that different levels of energy intake would alter the circadian rhythm of core body temperature (Tc) in ovariectomized sheep. We measured arterial blood temperature every 5 min while ten sheep were offered a maintenance diet, 70% of maintenance requirements, or 150% of maintenance requirements, for 12 days, and later fasted for 2 days. The rhythmicity of Tc was analyzed for its dominant period and then a least-squares cosine wave was fitted to the data that generated a mesor, amplitude, and acrophase for the rhythm. When energy intake was less than maintenance requirements we observed a significant decrease in the mesor and minimum, and a significant increase in the amplitude and goodness of fit, of the body temperature rhythm. Fasting also resulted in a decrease in the maximum of the body temperature rhythm. Feeding the sheep to excess did not affect the mesor or maximum of the rhythm, but did result in a decrease in the goodness of fit of the rhythm in those sheep that consumed more energy than when they were on the maintenance diet, indicating that circadian rhythmicity was decreased when energy intake increased. Our data indicate that modulation of the circadian rhythm of body temperature, characterized by inactive-phase hypothermia, occurs when energy intake is reduced. The response may be an adaptation to energy imbalance in large mammals.