Is 眠たい an adjective?

Question

I was told 眠たい was an adjective, and that it is an exceptional case. How does it work as an adjective and what is its relation to the verb 眠る?

Answer 1

Yes, 眠たい is an adjective. It's a synonym of 眠い, meaning "sleepy." (大辞泉 entry here.)

From what I can find (specifically, 大辞林's explanation), 眠たい is in fact related to 眠る, but the 「〜たい」 here is different from the desiderative "I want to" 「〜たい」. This 「〜たい」 is a shortened form of 甚{いた}い (or いたし, in its original form in older Japanese), which is a 連用形 verb ending in classical Japanese meaning "extremely X, where X is the state related to the verb." Here, this "state" is sleepiness. Note that this 甚{いた}い can attach to verbs and give them an emphatic adjectival meaning, and does not attach to adjectives.

めでたい (happy, fortunate) has a similar etymology: 愛{め}で (from the 連用形 of 愛{め}ず, the old form of 愛する) ＋　甚{いた}し.

Edit: Just to clarify, the original form of 眠たい was 眠{ねぶ}り甚{いた}し, but a combination of phonetic drift and other factors led it to eventually evolve into its current form.

Answer 2

It is an i-adjective because it conjugates as so.

眠たい, 眠たかった, 眠たくない, ...

Its relation to the verb 眠る cannot be handled systematically within derivational morphology. The stem of 眠る is nemur-, whereas 眠たい only uses a portion (nemu-) of it. So it has to be considered that this formation is idiosyncratic, not systematic. Note that the meaning is also idiosyncratic.