For two whole nights, the doctor has sat up with the gentlewoman simply to be able to witness Lady Macbeth sleepwalking to better diagnose her illness. Believing that his efforts to see Lady Macbeth in this unusual state may be in vain, he attempts to gain information from the gentlewoman by asking her, "[w]hen was it she last walked" (5.1.2) and "what at any time have you heard her say" (5.1.11). Yet, the doctor does not give up after two sleepless nights; he instead shows that he is concerned and devoted to the Queen by faithfully continuing to wait with the gentlewoman to observe Lady Macbeth sleepwalking.
When Lady Macbeth does enter, her illness appears to exceed the doctor's realm of colds and fevers, as he realizes that Lady Macbeth is dealing with a problem that is causing her much mental anguish. When Lady Macbeth cries, "[w]hat need we fear? Who knows it, when none can / call our power to account? Yet who would have thought the old / man to have so much blood in him" (5.1.32-34), she reveals to the doctor that she aided in murderous acts that are causing her such pain. The doctor is shocked at her confession and disbelievingly says to the gentlewoman, "Do you mark that?" (5.1.35). With the murders that have been committed in Dunsinane Castle recently, the doctor interprets Lady Macbeth's speech to be an admission of her part in the deaths, specifically that of Duncan, whom she refers to as the "old / man"(5.1.33-34). The doctor laments, "[m]ore needs she the divine than the physician" (5.1.64). He now understands the root of Lady Macbeth's decline in mental health and sleepwalking. He has come to the conclusion that the Church and God would better deal with her problems and that none of his remedies can aid her. With this troubling new information about Lady Macbeth, the doctor could reveal it to others and therefore turn her in. Nevertheless, he resolves to "think, but dare not speak" (5.1.69). The doctor is resolute in his decision to be loyal to Lady Macbeth about whom he cares. He thus concludes not to reveal to anyone else his discovery that it was Lady Macbeth, at least in part, who was involved in the murders that have been occurring in Dunsinane Castle.