Compared to its sister poem, “L’Allegro”, “IL Penseroso” is a somber and reflective take on the maturing individual. It is actually almost a depressive piece, were it not for the end which invigorates the speaker will a will to power (“These pleasures melancholy give, / And I with thee will choose to live” Lines 175-6).
The ending lines of “L’Allegro” read “These delights if thou canst give, / Mirth, with me, I mean to live” (Lines 151-2). Then, immediately at the start of “IL Penseroso”, we read “Hence vain deluding joys” (Line 1). Obviously this is a direct response, as well as an attack—as in, an assault on the idea of exploring the world without preparing one’s self intellectually and spiritually— on its sister poem; but why the hostility?
If the previous poem was concerned with worldly wonders and living life to the fullest—being sociable and experimenting (or, Day, as Hughes calls it in the introduction), then the interlocutor of Day, Night, will be on the reverse track: in other words, silent, religious, contemplation. Whereas Day is able to allow people to travel, socialize, and build the future, Night is for final meals, repentance, and quiet consideration of matters both personal as well as religious.
“Where glowing Embers through the room / Teach light to counterfeit a gloom” (Lines 79-80). Here, Milton belays the idea that anything other than spiritual enlightenment is authentic; indeed, earthly light is an allusion (or, so sayeth, at any rate, the previous owner of my textbook; I happen to agree with them, however). So, when we read some more—“Hide me from Day’s garish eye” (Line 142)—we learn that the Goddess speaker, and I suppose we can call he a goddess in the pagan sense, unless Milton instead wanted readers to look at her as the Virgin Mary, we learn that the speaker is not melancholic in the modern sense—sad, depressed—but melancholic in the academic sense; this speaker has foreclosed on herself in order to live the reflective, religious life (“To walk the studious Cloister’s pale” Line 156).
Ultimately, this is what characterizes Night—a time for private issues to be grappled with and personal enrichment to be wrought. Day can never give that, not when you struggle to make a living and do what needs to be done to make that living. No. It is only in the nighttime that our Goddess speaker, perhaps even more so because she is a woman, can further herself and move on to a higher existence—Christianity if she is a pagan stand-in, and union with God Itself if she is a Virgin Mary stand-in. In either case, the situation is clear, and that is that Day and Night are one in the same, but directed toward different ends in God’s master plan.