How do the shifts in setting from the blazing heat of day to the cold night on the lagoon provide a kind of commentary on arsat’s tale?

The correct answer is:  

The shifts reinforce the emotional extremes in Arsat's tale.

In "The Lagoon" (1896), a short story by Joseph Conrad, "Tuan" (meaning Lord or Sir) is a white man traveling through a rainforest in Indonesia. He will have to stay for one night with his remote Malay friend named Arsat, who is distraught because his lover is dying.

he is suggesting that this widow is a strong woman, who is giving constructive criticism and she has enough to worry about, and she takes care of him, so he does not take it to heart.

adding more details about the struggle rainsford experiences as he swims to the shore