Three short lines carry a surprising amount of unease. The poem pictures a late night, and something is "eating at" it: either a bottle or a book. Those two things are the usual companions of a sleepless person, one for numbing the mind and the other for occupying it. The speaker doesn't choose between them, which suggests the difference hardly matters. Either way, the night is being used up, and perhaps the speaker along with it.
The second stanza turns that feeling into something bigger. The force at work is compared to a god the speaker can neither guess at nor believe in. That is an odd, uncomfortable position: no faith to lean on, but no comfortable certainty either. The title, "Inscrutable," fits this. The speaker senses something large and unreadable pressing on the dark hours and can't name it.
The mood is quiet, restless, and a little bleak, but it's honest rather than dramatic. The poem leaves the question open.