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.

The lovers in this poem encounter their own shadows. The shadows are a metaphor for the way we hide parts of ourselves from a beloved. We often do so before the love has fully blossomed, at which point we feel secure. At that moment, the high point of love, we reveal ourselves fully, without shadow, to the other person.

Donne's speaker compares this fullest point of love to noon, the time of day when all shadows disappear. He says that, like noon, it can pass very quickly. Lovers can start to disguise things or keep secrets from each other, and this can cause the fullest love to fade.

Lovers need to be mindful that love will "decay" if they aren't fully open and honest with each other. True love, the speaker implies, takes hard work and a complete commitment to transparency. Otherwise the shadows will deepen and deepen until the light of love darkens into night.

Because of the difficulty of staying in the "noon" of love, the poem can seem pessimistic. True love is depicted as fleeting, like noontime, lasting only for a brief moment. Yet by talking to his beloved frankly and realistically about the need to strive to keep love at the high point of "noon," the speaker increases the possibility that their love will flourish.