Imagine the whole universe as a restless sleeper. That's the picture at the heart of this poem. Space is a "vacuum bed," and the Universe, written with a capital letter and the pronoun "Its," tosses and turns through a night that never seems to end. Now and then it wakes, opens its eyes, and light pours in and out of them. We can read those bright moments as stars, galaxies, or any burst of cosmic brightness, though the poem never says so directly. Then the Universe slips back into darkness and unconscious dreams.
The speaker sounds quiet and awed rather than frightened. The heavy repetition of "long long night" works like a slow lullaby, and it makes the darkness feel endless. Light is rare and brief, while the dark is vast and patient. The mood is calm, a little lonely, and strangely tender, as if the speaker were watching over something enormous and tired.