What is difference between somnolence and sedation?

Answer 1

Hypersomnia is a general symptom and a category of sleeping conditions for those who have problems staying awake (more sleepy). Hypersomnia is experienced by those with narcolepsy and hypersomnolence (which are separated between those who experience R.E.M. and those who do not).

Short answer: hypersomnolence is hypersomnia, but hypersomnia covers more than hypersomnolence like narcolepsy.

Example: Invasive ductal carcinoma is a Breast cancer, but not all cancers are breast cancers. Hope this clears it up, if not feel free to correct me.

Answer 2

From Wikipedia:

Somnolence (or "drowsiness") is a state of near-sleep, a strong desire for sleep, or sleeping for unusually long periods (cf. hypersomnia). It has two distinct meanings, referring both to the usual state preceding falling asleep,[1] and the chronic condition referring to being in that state independent of a circadian rhythm. "Somnolence" is derived from the Latin "somnus" meaning "sleep."

Sedation is the reduction of irritability or agitation by administration of sedative drugs, generally to facilitate a medical procedure or diagnostic procedure. Drugs which can be used for sedation include propofol, etomidate, ketamine, fentanyl, and midazolam.[1]