sleep pressure
Technical term in sleep science referring to the homeostatic drive for sleep that builds up during wakefulness.
睡眠圧が高まると、自然に眠くなる。
When sleep pressure builds up, you naturally feel sleepy.
Compound of 睡眠 (suimin, 'sleep') and 圧 (atsu, 'pressure'), likely a calque from English 'sleep pressure'.