When niacin/NAD⁺ grows insufficient, two things happen that both work against the free-tryptophan pool available for serotonin→melatonin:

1. Supply side (delivery of free Trp falls).
   Unaddressed continued lack but growing demand for nicotinic acid damages the gut and impairs exocrine pancreas (smaller gland, lower trypsin/chymotrypsin/amylase stores and secretion). That reduces protein digestion and amino-acid absorption, so less free Trp appears in plasma from a given meal.
2. Demand side (diversion of Trp rises).
   A low NAD⁺ state disinhibits the de novo pathway (quinolinate → NaMN) and, with any inflammation or stress, IDO/TDO push Trp down the kynurenine pathway. Net effect: a larger fraction of a smaller Trp pool is consumed to rebuild NAD⁺, leaving less for serotonin and melatonin.

This process continuing to ensue unchecked defines aging and disease.