In "The Adventure Of the Speckled Band" why did Helen Stoner hear the whistle when she did?

Helen Stoner had never heard the whistle until the night before she came to consult Sherlock Holmes. She tells him:

Two days ago some repairs were started in the west wing of the building, and my bedroom wall has been pierced, so that I have had to move into the chamber in which my sister died, and to sleep in the very bed in which she slept. Imagine, then, my thrill of terror when last night, as I lay awake, thinking over her terrible fate, I suddenly heard in the silence of the night the low whistle which had been the herald of her own death.

Helen had not heard the low whistle before because her bedroom was separated from her stepfather's room by Julia's bedroom. Julia had described the whistle to her sister shortly before her death.

"...during the last few nights I have always, about three in the morning, heard a low, clear whistle. I am a light sleeper, and it has awakened me."

Julia would have been likely to hear the low, clear whistle because it was coming from right next door. There was also the fact that she was "a light sleeper." And furthermore, there was a ventilator between Julia's room and Dr. Roylott's which would make it easier for the sound to carry. Helen is apparently a heavier sleep than her sister was. When Julia seemed surprised that Helen hadn't heard the whistling, Helen replied:

"Ah, but I sleep more heavily than you."

Dr. Roylott had to make his whistling loud enough for the snake to hear it but not so loud that it would wake Julia. Helen associated the whistle with her sister's death, and Helen heard the whistling almost as soon as she was forced to move into her dead sister's room. Helen did not hear the whistle on the first night she slept there, either because she slept through it or because Dr. Roylott waited one night before sending his snake through the ventilator. If Roylott waited one night before attempting to kill Helen, it might have been because he thought it advisable to let her get accustomed to sleeping in that bedroom which had such unpleasant associations for her.

Helen describes hearing the whistle for the first time:

Imagine, then, my thrill of terror when last night, as I lay awake, thinking over her terrible fate, I suddenly heard in the silence of the night the low whistle which had been the herald of her own death. I sprang up and lit the lamp, but nothing was to be seen in the room. 

The poisonous snake must have been right beside her on the bed when the whistling began. By the time Helen "sprang up and lit the lamp" the snake would have been climbing up the dummy bell-rope. Helen would not have seen it because she would not have thought of looking up in that direction. The snake was small and probably quick in its movements. It must have been well trained to respond to the whistle, which meant a reward of a saucer of milk.