Rainsford is having trouble sleeping. He cannot rest due to his anxiety from learning that Zaroff hunts human beings. During dinner, Zaroff had disclosed this information about hunting humans. This bizarre action causes Rainsford to stay awake during the night. He is thinking about what Zaroff told him during dinner:

While the men are eating, Zaroff reveals his passion for the hunt. He tells Rainsford he hunts "big game" on the island—game he has imported. Hunting had ceased to be a challenge to Zaroff, so he decided to hunt a new animal, one that could reason. Rainsford realizes with horror that Zaroff actually hunts humans

Now, Rainsford is lying awake. He cannot sleep. Zaroff has shared his strange behavior which involves hunting humans. Rainsford wonders what will happen if a human being refuses to be hunted by Zaroff:

[Rainsford] wonders what happens if a man refuses to be hunted. He finds there is no refusing Zaroff, for either a man goes on the hunt or he is turned over to the brutish Ivan. Zaroff never loses. Although Rainsford passes the night in comfortable quarters, he has trouble sleeping. As he finally dozes off, he hears a pistol shot in the jungle.

No doubt, Rainsford has spent a sleepless night thinking that he may be the next victim of Zaroff's hunt. He plans to refuse Zaroff. He plans to demand to be taken back to the mainland. Zaroff has other plans and refuses to honor Rainsford's wishes:

The next day Rainsford demands to leave the island. Zaroff protests that they have not gone hunting yet, then informs Rainsford that he, in fact, is to be hunted. Zaroff tells him that if he survives three days in the jungle, he will be returned to the mainland, but he must tell no one of Zaroff's hunt.

Zaroff hunts Rainsford for three days. Truly, Rainsford is exhausted from the hunt. Ultimately, Rainsford survives the hunt. In the end, it is presumed that Rainsford kills Zaroff for Rainsford sleeps in Zaroff's bed. He states that he has never had a better night's sleep:

That night, Rainsford sleeps with immense enjoyment in Zaroff's comfortable bed.

In "The Most Dangerous Game," why can't Rainsford "quiet his brain" after meeting Zaroff?

As a seasoned hunter, Rainsford of "The Most Dangerous Game" is disturbed by the reflections of Zaroff that he has become jaded by hunting wild game.  When Zaroff states that he "had to invent a new animal to hunt," Rainsford is taken aback:  "A new animal? You're joking."  Rainsford here  suspects that Zaroff's reply is going to be bizarre.  But he is bewildered and then appalled.  When Zaroff suggests that the "new animal" must be able to reason, Rainsford gasps, "But you can't mean--"

Zaroff then explains that his prey comes to him because of his island which is aptly name "Ship-Trap Island."  He blandly describes to Rainsford his "game" what he has with captives from shipwrecked vessels.  After this conversation, Rainsford excuses himself, "I'm really not feeling at all well."  Zaroff suggests that they hunt on the morrow:  "I've one rather promising prospect--"

In bed, Rainsford can not sleep.  He is wondering if he will become the prey for tomorrow's hunt since he could be the "promising prospect" of whom Zaroff speaks; afterall, he is an renowned hunter himself and would provide quite a challenge to the sadistic general.  With this thought in mind, Rainsford could easily wonder where the hunt would begin, what type of terrain lies outside the chateau, how he would defend himself, where he could go, what he would do even if he did kill Zaroff.  Rainsford gets up and goes to the windows.  All is dark, but the vicious dogs hear him and look up "expectantly, with their green eyes."  Will Rainsford be their next victim?  With only a disturbed sleep Rainsford rests uneasily in a doze until he is awakened by "the faint report of a pistol."  Rainsford knows that either Zaroff or one of the captives has been killed.