Insomnia
Etymology
From Latin “in-” (not) and “somnus” (sleep) — straightforwardly “sleeplessness.” “Somnus” was also the Roman god of sleep, and the root survives in “somnolent” and “somnambulist.” The word’s flat literalness is fitting for a condition defined by simple, grinding absence.
noun · /in-SOM-nee-uh/
Persistent difficulty falling asleep or staying asleep, despite the chance to do so.
What it names
Sleeplessness as a genuine condition rather than a passing bad night, quietly insisting on the crucial qualifier — the opportunity to sleep is there; the sleep is not.
Example
Months of insomnia left her staring at the ceiling until three each morning, exhausted but wide awake.
Related idea
Insomnia is the inability to sleep when you have the chance; sleep deprivation is the shortfall that follows when you have no chance — a new parent or a night-shift worker is deprived, not insomniac. The distinction is between a broken mechanism and a stolen opportunity.