My older hens usually don't start laying again until Jan sometime. The daylight needs to start increasing in order to trigger their egg laying hormones.
Exactly.
It is day length but more importantly, they need to detect a gradual change in the light/dark cycle and the length of each.
I wrote the following in another thread and it may explain a bit more of the science behind light.
Light exposure to the retina is first relayed to the nucleus of the hypothalamus, an area of the brain that coordinates biological clock signals. Fibers from there descend to the spinal cord and ultimately project to the superior cervical ganglia, from which neurons ascend back to the pineal gland. The pineal gland translates signals from the nervous system into a hormonal signal.
The gland produces serotonin and subsequently melatonin, a hormone that affects the gonads for sperm production in males and ovulation in females. An increase in melatonin causes the gonads to become inactive. As photoperiod in relation to day vs. night is the most important clue for animals to determine season. As it lengthens, the gonads are rejuvenated. The duration of melatonin secretion each day is directly proportional to the length of the night because of the pineal gland's ability to measure daylength. Besides reproduction, it also affects sleep timing and blood pressure regulation.
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