Tired is a survivor hindrance that results from a lack of proper sleep or excessive physical labor, which can be results of various Events.
When a party member is tired, their combat stats (strength, fitness, and shooting) take a temporary reduction, usually 3 or 4 levels to a minimum cap of -1, indicated by a flashing unhappy icon.
While scavenging, if a character expends their energy earlier than usual from swinging a weapon, a blue icon with an exasperated face will appear over them as a reminder. Tiredness will disappear once another day ticks over (assuming the character does not have trouble sleeping). This is indicated by the consumption of Food often accompanied by an Event.