To what metaphorical "sleep" might the speaker be referring in the last lines?
In the poem "Stopping By Woods on A Snowy Evening" he poet Robert Frost describes how a winter traveller becomes momentarily spellbound by falling snow against a background of woods which appear to be filling with it. This appears on one level to be like a longing for silence, endings, for being covered and obliterated. Some critics have seen in this an illustration of graves, of coffins, of death. However, death is not the only solace in times of stress and exhaustion. Sometimes human beings just need to take time out,as the traveller does, for a short while - a respite. Often, stressed professionals take time out for meditation, relaxation classes - for resting the mind just as Frost describes here. Nature itself can be visually restful - and is enough to recharge the batteries so we can go on.
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?”