Sunset. Twilight. Dark. Yep, that's pretty much how it goes when you die. First, you grow a little older, a little crustier. Then you grow really old and crusty. And finally, you reach the point of no return—full on nighttime. At least, that's how our speaker sees it.
Line 1: The speaker opens the poem by talking about "sunset and evening star." It's the first night imagery, and it sets a tone for what will soon become a whole slew of metaphors comparing death to the coming of night.
Line 5: When the speaker mentions sleep here, we can't help but think of nighttime.
Line 9: "Twilight and evening bell" both refer to a time of day, but, like the images in line 1, they're also metaphors for the speaker's advanced age and impending death.
Line 10: After twilight, there is nothing but "the dark." Scary, right? This creepy little line is a reminder that we have no idea what's coming when it comes to death. We're totally in the dark—pardon the pun.