Duh. Caffeine mimics serotonin. Smoking boosts niacinamide (aka nicotinamide, vitamin B3) signalling.

The two main paths for tryptophan metabolism are [majority] niacinamide/kynurenine synthesis and [minority] serotonin synthesis.

Heavy niacinamide/kynurenine demand diverts tryptophan from serotonin production, leads to chronically low serotonin.