There is a strong connection between the green light at the end of the dock and this passage. The moment of extreme delight as Daisy examines his bedroom represents the high point of the overwhelming joy Gatsby experiences at finally achieving his five-year-long dream of reuniting with Daisy. He shades his eyes because, as Nick explains, he has been running at "an inconceivable pitch of intensity." Daisy is like a brilliant sun to him, and he needs to shade himself from the reality of her presence, which is suddenly so close.

But this is the high point. As Nick explains,

in the reaction, he was running down like an overwound clock.

A little while later, having shown Daisy the house, Gatsby and Daisy, with Nick tagging along, tour the grounds. Gatsby mentions to Daisy that if the mist weren't there, they would be able to see the green light at the end of her dock. Nick understands the poignance of the moment. He states, in a famous passage,

Possibly it had occurred to him that the colossal significance of that light had now vanished forever. Compared to the great distance that had separated him from Daisy it had seemed very near to her, almost touching her. It had seemed as close as a star to the moon. Now it was again a green light on a dock. His count of enchanted objects had diminished by one.

What Nick understands is that achieving our dreams inevitably tarnishes them. A reality, no matter how wonderful it is, can't match our imagined dream of it. Now that Gatsby has Daisy back in his life, she will inevitably became more real and less enchanted. She is no longer the longed-for green light at the end of the dock, but a real person. The high point of a dream, Nick is saying, comes the moment we realize it—everything else is downhill.