Details on the neuro-chemistry:
The mechanism for translating light into mood starts in the suprachiasmatic nucleus*, which sits right behind where your optic nerves cross.
Light-sensitive nerves trigger and transfer that information to the pineal gland, which in turn, starts breaking down serotonin to melatonin.
I dream about a brain implant that artificially stimulates the SCN. Control that stimulation and you control sleep, serotonin, melatonine, hunger, cell-growth, stress etc. etc. etc.