涼しい and 蒸し暑い: a cool time for work and a muggy night
Read a cool breeze and 涼しいうちに alongside a hot, humid room and night. Preserve the time condition for beginning fieldwork and the past act of sleeping with the window slightly open.
A cool breeze and a time still cool
涼しい（すずしい） means cool. 「涼しい風が窓から入ってきました。」（A cool breeze came through the window.） The breeze coming through the window is cool. The sentence does not add an evaluation of unpleasant cold.
「暑い日は涼しいうちに畑の作業を始めます。」（On hot days, we begin working in the field while it is still cool.） 涼しいうちに gives the time condition of while it is still cool. It describes when fieldwork begins on hot days, rather than reporting work already completed now.
A hot, humid room and night
蒸し暑い（むしあつい） means hot and humid or muggy. 「雨上がりの部屋は蒸し暑かった。」（The room was hot and humid after the rain stopped.） The room after the rain stopped is the target. Retain the humidity in the meaning rather than reducing the adjective to temperature alone.
「蒸し暑い夜なので、窓を少し開けて寝た。」（It was a muggy night, so I slept with the window slightly open.） 蒸し暑い夜なので gives the reason, and 窓を少し開けて寝た describes the past act of sleeping with the window slightly open. Preserve 少し without making that act a recommendation for every night.
The state and the action's scope
The 涼しい examples give a cool breeze and a time still cool. The 蒸し暑い examples give a hot, humid room after rain and an action taken on a muggy night. Read the difference between coolness and humid heat.
畑の作業を始めます is tied to the time of beginning work, while 寝た reports sleeping on a particular night. Keep the time condition and past action distinct without supplying numerical temperature or humidity limits.
What states and action scopes appear in 「暑い日は涼しいうちに畑の作業を始めます。」 and 「蒸し暑い夜なので、窓を少し開けて寝た。」?
Show the answer and why
The first gives the condition of beginning fieldwork while it is still cool. The second reports sleeping with the window slightly open on a hot, humid night. Humidity remains part of 蒸し暑い, and the action does not become a general recommendation.