Silene antirrhina is a slender plant growing to 30-inches tall. Small white flowers, about 1/4-inch across, have deeply notched petals. The calyx below the flower is glabrous (hairless) and is not inflated, in contrast to most other Silene spp. Leaves are opposite and narrow. Sleepy Catchfly has bands of sticky resin on the upper stem, appearing red in the photos. Insects sometimes get caught in the resin.
Compare to Melandrium dioicum, which has larger flowers (about 1-inch diameter) and an inflated, pubescent calyx. Melandrium dioicum is dioecious.