If you're somnolent, you're feeling sleepy or drowsy. It's best to avoid operating speedboats or motorcycles when you're somnolent.
Somnolent comes from the Latin word somnolentia, meaning sleepiness, which in turn is from the Latin root somnus, for sleep. You can feel somnolent, or describe something as somnolent. As an adjective it describes something that is likely to induce sleep, like a boring movie in an overheated theater, or the low, somnolent lighting in a museum exhibit of fragile, old illuminated manuscripts.