не в себе BUT как не в себя?

Question

According to Wiktionary, the standard forms are:

• не в себе
• как не в себя

There's an inconsistency in the endings. Is it correct that one uses себе and the other uses себя?

Google Books Ngrams shows как не в себя used to be very rare compared to как не в себе, but rapidly caught up in the last couple of decades, and they are now equally frequent.

As far as I know the first one is used to mean that a person is feeling a bit "out of sorts" or not in their right mind, while the second one can also be used to refer to inanimate objects:

• Будильник звонил, как не в себя = The alarm clock was ringing like crazy

Answer 1

Как не в себя is a humorous set phrase which literally means "as if not into oneself", or, elaborating on this, "to eat (drink, etc.) as if the food were not going into your body".

It does mean "to eat like a horse, to drink like there's no tomorrow" and so on. By extension, it can be used as an adverbial phrase meaning "a lot, like crazy" and similar, as you can see in your example.

Another similar expression is как из ружья. It started as a part of the metaphoric expression выпалил, как из ружья meaning "blurted out", but then took on a life of its own, and now just means "very much" (about wanting something):

• [Валерик (муж)] Есть охота! Как из ружья! В «Арагви», что ль, махнуть, поужинать? [М. М. Рощин. Старый Новый год (1967)]
• На душе тоскливо. Ночь. Спать хочется как из ружья

Не в себе means "out of one's mind", both literally and metaphorically.

Answer 2

@Quassnoi has explained the meaning, but I would highlight the main reason why the endings are different: cases.

The preposition "в" can be used with different cases depending on the meaning. In your case, the two phrases have different meanings, and thus use different cases:

не в себе

this is meaning 1 according to the link above:

[with prepositional] (location) in, at, on

It refers to a location, means that something is (or isn't) somewhere. "Out of one's mind", as @Quassnoi puts it. Therefore, the prepositional case is used. Compare "он не в театре", "вода в кастрюле", etc.

как не в себя

this is meaning 2.1 according to the link above:

[with accusative] (direction) to, into

It refers to the direction of some movement, that something is moving or being moved (into) somewhere. "As if not into oneself", as @Quassnoi puts it. Therefore, the accusative case is used. Compare "он идет не в театр", "наливать воду в кастрюлю", etc.

Compare also the phrase "выйти из себя" ("get out of oneself", "get out of one's mind"). While the allusion is the same as in "не в себе", this phrase refers to the direction of movement, not to a static location, and therefore uses the accusative case.