Impairment
Since handling a firearm is a complex task, with possible fatal outcomes if done improperly, gun safety dictates that a firearm should never be handled while under the influence of alcohol or drugs, or legal prescription or over-the-counter drugs. Since such substances may affect a person's judgment even after consuming relatively small amounts, zero tolerance is advocated by gun safety teachers. This is codified in many states' penal codes as a crime of "carrying under the influence", with penalties similar to DWI/DUI.
Exhaustion can also constitute a form of impairment, as reaction time, cognitive processing and sensory perception are all impaired by sleep deprivation and/or physical exhaustion. Gun safety therefore discourages using firearms when exhausted.
Read more about this topic: Weapon Handling