The apple-picker is mentally exhausted. He is on the edge of sleep, but his mind is still very active, and he knows "what form (his) dreaming is about to take". He is physically tired, but his mind is fatigued as well - he has "had too much of apple-picking...(he) is overtired of the great harvest (he) himself desired". His sleep will be troubled, "whatever sleep it is", because his mind cannot find respite.
The poem is clearly allegorical. Concretely, it describes the malaise of the apple-picker, but critics believe that the mental fatigue which is its central theme applies to the poet's experience as a writer as well. "After Apple-Picking" was written immediately after Frost's first book came out, a book which he had difficulty getting published and which received little acclaim. The feelings expressed in the poem might express Frost's own sense of exhaustion after his initial endeavor, and his doubt about his future as a poet.
The message of "After Apple-Picking" can be applied to any field which a person loves, but expends a tremendous amount of energy in which to achieve a goal. In any such area, a "great harvest" is much desired, but the person is left mentally drained after having realized it. After reaching this point, the resulting fatigue can sometimes be remedied by "just some human sleep", but other times will require a longer respite, a hibernation, like the "long sleep" of the woodchuck.