first thing in the morning
Refers to the very first activity or time period after waking up, often implying early morning. Used as a noun or adverbially with に.
I check my email first thing in the morning.
I take the first Shinkansen in the morning.
I make it a point to take my medicine first thing in the morning.
I check my email as the very first thing in the morning.
Compound of 朝 (morning) and 一番 (first, number one). The exact historical derivation is uncertain, but it naturally expresses the idea of 'the very first thing in the morning'.