I think the Nightwatcher gives just one thing normally, but that thing is both boon and curse. Ex: Dalinar driven nearly insane by the death of his wife ask for his grief to be healed. So the the Nightwatcher heals it by removing all his memory of her and thus his grief. Tarvangain asked for the capacity to save the world and so his mind focus more completely on one aspect of human conciseness. He never dumber or smarter just more or less analytic. Normal Human mind is say: 25% , 30% empathy and relationships, 15% standard operation (eating, clothing, speaking, etc.), 20% observing and handling the immediate environment and situation, 10% fight/flight decision. But Tarvangain it's more random so what he thinks are his best days are just days when he is 90% long term problem analysis and solving and 10% for everything else. single changes that lead both boon and curse. All based on the old saying "When God wish to punish us the most, he answers are prays."