Concept:The poem is built on nostalgia — a heartfelt longing for happier moments from the past.Explanation:The speaker remembers childhood times and friends who are no longer present.These memories arise during the night and bring feelings of sadness.The central mood is not simply about being awake at night or about romantic loss.It is a gentle sorrow caused by remembering joys that can never come back.Thus, the poem mainly expresses a deep desire for happier days that have already gone.Answer:D. yearning for happier times gone