The simple subject in the sentence is "nurse".
A subject in a sentence tells who or what is performing an action. Subjects are classified in simple or complex based on weather they have modifiers or not.
In a simple subject, you have to identify the main word or the performer of an action within the subject, without considering the modifiers.
In this case, the simple subject is "nurse" without taking into account the modifiers "the tired".
A complex subject is "The tired nurse".