In stadium (or at least in the stadium practice range), when the “Dreamy” power, “Comfy Cloud” power and “Sleep Regimen” powers are all equipped together with the “Tranquilizer” Hero Upgrade, it reduces sleep dart damage to 0. This includes the extra damage from “Lethal Dose” hero item upgrade. This effect is not listed in the description of the upgrade, so I assume this is unintended. It also seems to stay that way even after the “Tranquilizer” power is sold (in some cases, it’s really inconsistent).
This post was edited multiple times as I pinpointed the exact upgrades that created this interaction.