Hypnos was generally represented as a young, beautiful man – sometimes with a beard, but, in most cases, leaving the impression that he’s too young to have a beard yet – so, as you can see, he couldn’t be more different than today’s Sandman. The most common among the symbols of Hypnos are his characteristic wings, attached to his head, sometimes growing out of his eyebrows. Rarely, the wings are shown on his shoulders (not quite as exotic, I think you’ll agree).
Among the plants, the symbol of Hypnos is the poppy, whose seeds were famous for their sleep-inducing qualities. The god has sometimes a staff decorated with poppies in his hand, or a horn filled with poppy seeds or opium. Since sleep is the best way to forget the sorrows of the day, Hypnos was said to have his palace on the banks of river Lethe – the river of forgetfulness – and is shown at times carrying a branch dripping with water. The water comes from this river, and anybody coming in contact with it would forget everything and would be sentenced to wander aimlessly for the rest of his life.
Another one of the symbols of Hypnos – a bit less common – is a torch held upside down, symbolizing the darkest hours of the night, when people are fast asleep.
Hypnos is also easy to recognize in art because he’s rarely alone. Most often he’s accompanied by his brother (twin, by some accounts, or half-brother, by others) – Thanatos, the god of death, and they make a pretty disturbing pair together. Though Hypnos appears in the myths as a rather benevolent god, the constant association of sleep and death makes him quite ambivalent, if not downright spooky. He is after all one of the gods of the Underworld.
Hypnos is also accompanied by his sons, Morpheus, Phobetor and Phantasos – also known under the general name of Oneiroi, deities that can appear in the dreams of mortals.