In Andrea del Castagno's The Last Supper why is John asleep?

In Andrea del Castagno's "The Last Supper," John is depicted as asleep to convey his state of spiritual and emotional exhaustion. This portrayal emphasizes his innocence and vulnerability in contrast to the impending betrayal and turmoil surrounding the scene. Additionally, John's slumber can be interpreted as a visual representation of the disciples' inability to fully grasp the gravity of Jesus' impending sacrifice. This choice enhances the dramatic tension within the composition.