。
I tend to run a fever in the middle of the night.
Literal
Night-middle [in-に] fever [subj-が] comes-out-[explanatory-のです].
熱が出る ('develop a fever,' lit. 'fever comes out') is the standard collocation for running a temperature. 出る is used metaphorically for many bodily symptoms — 涙が出る ('tears come out'), 咳が出る ('develop a cough'). The ~のです frames the statement as explanatory: 'you see, I get fevers at night.'