From the 1994 book Ship of Dreams page 20.

Where do dreams go when morning comes and chases them from sight? Do they simply vanish into thin air? Certainly not! They simply drift back into the sky from whence they came. The Sandman gathers them up and returns them to their place in his overflowing collection of nocturnal clutter which he calls the Dreamer's Trunk. The trunk is full of the most extraordinary things like sheep for counting, a nightmare - and the Big Dipper which the Sandman uses to help sprinkle sleeping sand over all the world's children. Keep digging and you'll find nearly 100 dream-related items. - Dean Morrissey