Complex sentence: Like Sonnet 27, this poem, with its comparably unhappy ending, turns on the indistinguishability of day and night; they were both occasions of work in the former poem, but here they are both occasions of torture.
Split into simpler sentences: Like Sonnet 27, this poem focuses on the indistinguishability of day and night. However, in the former poem, day and night are both occasions of work or toil, whereas Sonnet 28 employs them as occasions of torture.