Step-by-step explanation:
The speaker is in his bed Through the open window, he can see the watchman walking up and down the dark and lonely street with a lantern in his hand. The speaker is resentful that he has to go to bed. He thinks that the watchman never goes to bed in his life. Hence he wishes to become a watchman and walk up and down in the dark street at night, having fun chasing the shadows with his lantern