Dr. Roylott waits until three o'clock in the morning before he releases the swamp adder and lets it crawl through the ventilator, as it has been trained to do. Holmes and Watson arrive in Helen's bedroom just after eleven at night in response to her signal light. She goes to sleep in the other bedroom, and the two men are alone. Watson writes:
How shall I ever forget that dreadful vigil? I could not hear a sound, not even the drawing of a breath, and yet I knew my companion sat open-eyed, within a few feet of me, in the same state of nervous tension in which I found myself. The shutters cut off the least ray of light, and we waited in absolute darkness.
The mood is one of nervous tension and suspense. They dare not make a sound for fear of alerting Dr. Roylott in his room right nextdoor. They don't know for sure that anything is going to happen that night. The mood is very similar to the one described in "The Red-Headed League" in the scene in which Holmes, Watson, Mr. Merryweather the banker, and a policeman are waiting in total darkness and silence for someone to break into the room where the boxes of French gold are stored.
Doyle was a very good writer. His Sherlock Holmes stories are still very popular today. He gets the reader to imagine that he is in these dark places with the detective and his companion waiting for something to happen, not knowing for certain when it is going to happen or whether it is going to happen at all.