You could also interpret this poem as a commentary on the way we present ourselves to others; as we develop relationships we are casting ourselves in shadow, hiding our true selves from the other person and presenting that version of ourselves that we want them to see, out of fear of rejection. But as the relationship grows we cast aside this shadow that trails behind us and allow the other person to see us as we truly are; the other person, likewise, reveals themselves in this noon sun, the height of the relationship. But,
Except our loves at this noon stay,
We shall new shadows make the other way.
As the first were made to blind
Others, these which come behind
Will work upon ourselves, and blind our eyes.
Unless the two people manage to hold their love at this high noon, new shadows will appear, this time in the opposite direction. And these shadows, instead of veiling one person’s true self from the other, will become inverted, and each of them begin to withhold secrets and harbor negativity that they do not want to other to see, harkening the end of the relationship – the night. Thus love is destined to wane, and once we have revealed ourselves in honesty to a partner, the next time the truth is concealed is the beginning of an inevitable decline.
"A Lecture Upon the Shadow" is a poem about love and, somewhat metaphorically, about vision. Using an extended analogy (a conceit), Donne's poem compares the vision of lovers (and the delusions of lovers) to a situation of light and shadow wherein sight is only unhindered at noon, when the light of the sun produces no shadow.
The meaning of the poem seems to be that when love reaches maturity it allows lovers to see one another clearly, without illusion (and so free from wishful thinking, from erroneous dreaming and fill-in-the-blanks romanticizing) that characterizes immature love. When the maturity of love fades (or the strength of love fades), the lovers' vision again becomes compromised.
Only when love is at its pinnacle does it produce clarity of sight.
But, now the sun is just above our head,We do those shadows tread,And to brave clearness all things are reduc'd.
True love then means that lovers are both willing and capable to love the other person for who they are and to leave off any creative tricks of vision--to see the beloved as he or she truly is.
Love takes on the functional role of providing one with the power to see clearly, although this function exists in a somewhat circular context. Love provides the courage to see without illusion but the lack of illusion is also the definitive characteristic of true, honest love.
When love wanes to a point of lesser power (anything less than full strength), it ceases to provide the courage of honesty and ceases to clarify.
But oh, love's day is short, if love decay.Love is a growing, or full constant light,And his first minute, after noon, is night.
John Donne's "A Lecture Upon the Shadow" is about the length of a person's love life--it is too short.
The “cares” were fears and uncertainties; the “disguises” were the pretenses that the lovers put on so that others wouldn’t know they were in love. They are diligent in this, with love “still diligent lest others see.” These fears disappear the way shadows disappear under their feet under the hot clarity of the noonday sun. The new shadows (fears) that appear in the afternoon are different from those experienced in the morning. The morning shadows stand for fears about others knowing of their love; the afternoon shadows stand for fears about the other’s loyalty and sincerity. (“These which come behind / Will work upon ourselves, and blind our eyes.”)
The last line of the poem says that the “first minute after noon is night.” While the morning’s fears are short-lived, the afternoon shadows would normally grow longer throughout the day. If the lovers allow these to exist and their love to “decay,” their day will turn to night quickly. This perspective clips the usual cycle morning—noon—afternoon—night; it becomes morning—noon—night.
John Donne’s “A Lecture Upon the Shadow” uses the extended metaphor of sunlight to describe love’s fleetingness. The stanzas consist of thirteen lines and follow an abnormal rhyme scheme, each beginning with a couplet. It also takes the form of a meta-poem, as the speaker addresses the reader directly:
“I will read to thee
A lecture, love, in love's philosophy” (lines 1-2).
The poem centers on the noon hour, mentioning noon twice in the second stanza as a reference point for the stages of love. Donne describes the moment when the sun is directly overhead, saying that at this moment, two lovers overcome the shadows of time and understanding;
“…now the sun is just above our head,
We do those shadows tread” (lines 6-7).
The first stanza has a tone of overcoming, as the lovers described see each other in perfect light at noon. The second stanza has a tone of foreboding, as shadows begin to envelope the subjects. As the poem progresses past noon, the speaker describes:
“We shall new shadows make the other way” (line 14) and
“Will work upon ourselves, and blind our eyes” (line 17).
The increased mention of shadows and blindness in the second stanza are meant to suggest a growing distance between the lovers. The poem culminates with the daytime of love turning to the darkness of night- this suggests love’s fleetingness and fatality;
“Love is a growing, or full constant light,
And his first minute, after noon, is night” (lines 25-26).