To what metaphorical "sleep" might the speaker be referring in the last lines?

The speaker is in familiar territory; he is riding his sleigh during an evening snowfall and has stopped to watch the woods "fill up with snow." There is nothing particularly noteworthy in the speaker’s decision to stop, for falling snow is lovely to watch. On another level, however, the stopping may signify a reluctance to move forward, a fear of the future, and so on. The speaker apparently feels embarrassed by the stopping, for he notes that his "little horse" must be taking exception to the action. The speaker seems to be projecting onto the horse his own ideas that we must be busy every second of our lives. In addition, the speaker has a sense of invading someone else’s property, for the "though" of line 2 suggests that he would not stop if the owner were present to observe him. Some commentators have asserted that the house in the village is a church, and that therefore the woody area belongs to God. The implications of this reading raise many speculations in a classroom full of students with varying religious backgrounds.

In the last stanza, the alternatives are brought into sharp contrast: the woods vs. the promises and the miles. The speaker opts for responsibility, involvement, and action; all this is embodied in the single word "but" in line 14.

Many readers who see in the poem a longing for death (“frozen lake,” “darkest evening of the year,” “The woods are lovely, dark and deep” seem to support this view), but that is not what the poem is exclusively about. If there is a momentary longing for death in the poem, there is also the reassertion of the will to face the tasks of living. As Frost put it, at the Bread Loaf Writers’ Conference in 1960, “People are always trying to find a death wish in that poem. But there’s a life wish there—he goes on, doesn’t he?”