Apparently not in any formal use, to use as an adjective for insomnia.

It does not appear in any online dictionaries, even Wiktionary. Some sources suggest that insomniac is used both as a noun (the person) and an adjective. There is also insomnious, which appears less frequently, also as an adjective, and presumably would have the adverb form insomniously but this is nowhere referenced.

*The word insomnial is only directly referenced in the Urban Dictionary, which means that it might one day be considered a word, rather than slang, if it receives sufficient use.

Just as was stated before: It can mean pertaining to insomnia.