Here's your Latin lesson for the day: somnus in Latin means “sleep.” If you tack the prefix “in-” (another Latin word, meaning “not”) onto it, you get a word that means “not sleep.” An insomniac, therefore, is a person who suffers from the inability to sleep.
Once you know the Latin root somn-, which you now do, then you will also recognize it in some related words—somnolent, meaning “sleepy, drowsy,” somniferous, meaning “sleep-inducing,” and somnambulate, meaning “to sleepwalk.”
By now, you can probably guess that a somnambulist is a sleepwalker. Perhaps the most famous somnambulist is Cesare, a carnival attraction in the 1920 silent film The Cabinet of Doctor Caligari.
We hope this story hasn’t been too somniferous—but, if it has, pleasant dreams!