Fasting (only fruits + milk) on Shivaratri is the classical langhana - one day of light eating, kept for the sake of the vigil rather than for its own sake, so that the night is spent awake and attentive instead of digesting. Bilva-patra is Shiva's own leaf: trifoliate, read as his three eyes and as the three gunas, and no Shivaratri abhisheka is counted complete without it. Its place in the rite is devotional, not medicinal - the leaf is offered because it is his, and nothing is claimed here about what it does to a body.