Phrases such as in the dark or the hive, or the dark of winter, etc. have been used here to describe when brood rearing starts. But there is some sunlight entering any hive whether the entrance is low or high, the hive manmade or building or tree, even if it's an observation hive. The observation or detection of any light by any bees in the cluster must be passed on to the rest somehow. So, photoperiod.

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