。
When I woke up, dawn was just breaking.
Literal
Eye [subject-が] awakened-[if/when-たら] night [subject-が] is-about-to-brighten place-[ところだった].
夜が明ける ('night brightens') is the set phrase for 'dawn breaks.' ~るところだった ('was just about to X') is one of the most useful aspectual frames in Japanese: ところ ('place') functions temporally here, pinpointing the moment right before an action was to happen. In the past, it means 'was on the verge of X-ing.'