In four quick lines, Millay’s speaker shrugs off a friend’s love of sleep. Go ahead, she says—let sleep cut each day in half if you want. But she’d rather stay awake and keep those hours for herself. If burning the midnight oil shortens her life, so be it: Time can take those missing years from the far end, from old age, not from the days she actually gets to use now.
The mood is witty and a little defiant, not gloomy. Sleep is called a dull knife, which makes rest sound bland and unappealing compared with whatever the speaker is doing in the small hours—reading, writing, living. The poem’s logic is deliberately cheeky: it treats a long life as less important than a wide-awake one.