Abstract
1. 1. A photoperiod-related seasonal rhythm in active period (scotophase), metabolic rate and core temperature was documented for animals held at 21.0 ± 0.1 °C ambient; animals that were habituated to long nights (10:14LD) had a greater metabolic reserve than those held in summer photoperiods (14:10LD). 2. 2. While relative weights of gonads and sex accessory tissues of mice show typical “winter” regression, interscapular brown adipose tissue mass was unaffected by photoperiod; moreover, IBAT beta adrenergic responses under “winter” photoperiods did not differ from “summer” photoperiods in the absence of cold stimulus. 3. 3. Thermogenic efficiency, measured as the increment of active period temperature level achieved per increment of active period metabolic effort, was highest for animals exposed to short photoperiods. Thermal conductance was reduced in animals exposed to short (10:14LD) photoperiods. Heat conservation and thermogenic response capacity was enhanced by melatonin treatment and short photoperiod.
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