Chapter VIII begins with this passage:
I couldn't sleep all night; a fog-horn was groaning incessantly on the Sound, and I tossed half sick between grotesque reality and savage frightening dreams. Toward dawn I heard a taxi go up Gatsby's drive and immediately I jumped out of bed and began to dress--I felt that I had something to tell him, something to warn him about and morning would be too late.
There is a strong sense of danger in the passage; Nick can't sleep because he feels that Gatsby is somehow endangered. Nick has "something to tell him, something to warn him about," something so important it cannot wait until morning.
The clue in the passage that suggests the reasons for Nick's distress is the reference to "grotesque reality" and Nick's "savage frightening dreams." To understand these references, this passage has to be considered in the context of what had just happened and what Nick had seen and heard.
On the drive back from the city, Daisy had run over Myrtle Wilson, killing her, while driving Gatsby's car. The sight of Myrtle's body had been shocking. Nick had told Gatsby shortly after the accident that his car had "ripped her open." This is no doubt at least a part of the "grotesque reality" that troubles Nick, but the bizarre reality of what he had witnessed extends further.
In a terrible irony, Gatsby's car driven by Tom's wife has killed Tom's mistress. Nick had been with Tom shortly after the accident and had observed Tom's behavior; Nick knows that Tom has been shaken by Myrtle's death and, very importantly, assumes that Gatsby had been driving. Tom had said to Nick, "The God Damn coward . . . He [Gatsby] didn't even stop his car."
Nick had also been troubled by another thought shortly after Myrtle's death:
Suppose Tom found out that Daisy had been driving. He might think he saw a connection in it--he might think anything.
Nick tries to imagine what Tom might be thinking. If he finds out that his wife had been driving, he might think that Daisy had run over Myrtle on purpose. Nick is deeply worried because Tom "might think anything," and Nick has no idea what Tom will do. This fear fuels the "savage frightening dreams" that Nick experiences.
Nick can't sleep because he is "half sick" with the shock of what he has seen and heard and with a terrible dread of what may come of it. He must warn Gatsby about Tom Buchanan and what Tom might do.