The poem is one long run of "after": talk of going to sea, drinking with a butcher and a violin, plates of bacon, capons and fritters, singing, and a dance or two with "my lord." Everything points to a loud, greedy, happy night. The title, "Fat Tuesday," fits that picture, since it names a day of feasting before a time of restraint.
Then the sentence turns. The speaker doesn't fall into a drunken sleep. Instead, "soberly and without fear," they lie awake. The final line is short and plain, and it lands with a thud after all that crowding abundance.
The speaker seems calm rather than upset. There's no panic in "without fear," but the quiet is serious. The mood moves from noise and appetite to stillness and reflection. Judging by the setup, the party may have left a space where thoughts of endings, or of the journey hinted at by "going to sea," can come in. The poem doesn't spell this out. It lets the silence do the work.