なつばて [natsubate] – Something about the 38℃ days just makes you tired, you know. In Japanese, that fatigue is called 夏バテ.
バテ is derived from （疲れ）果てる [(tukare)-hateru] which means “to become completely exhausted.” 夏 [natsu] means “summer.”
熱中症 [necchuushou] or “Hyperthermia” is the blanket term that refers to more serious heat-related ailments like heat stroke, heat cramps and real heat exhaustion.
Drink lots of water and if you’re up to it, try some eel.