Directly related to sleep:
* Somnolence: Drowsiness, sleepiness
* Somnolent: Sleepy, drowsy
* Insomnia: Difficulty sleeping
* Hypnos: Greek god of sleep (related to Somnus)
* Somnambulism: Sleepwalking
Figurative uses related to sleep:
* Somniferous: Causing sleep, sleep-inducing
* Somnifacient: A sleep-inducing medicine or drug
* Somnolent: Dull, sluggish (figuratively "sleepy")
Other less common words:
* Somniloquy: Talking in one's sleep
* Somnium: A dream
* Somnifuge: Something that prevents sleep
It's worth noting that many of these words are relatively uncommon in everyday language, but are often used in medical or scientific contexts.