Lack Of Sleep
noun, な adjective
Remember how 不足 is a physical shortage? Well, now you have a physical shortage of sleep. That's a lack of sleep.
ねぶそく
This is a jukugo word, which usually means on'yomi readings from the kanji. However, the first kanji 寝 actually uses its kun'yomi ね. 不足 is on'yomi as you'd expect, but the ふ in 不 changes to ぶ, so hopefully you've been getting enough sleep.
「今日も残業するつもりなの？」「えっと…ですね、今日は寝不足なのでどちらかというと早くあがりたいと思っています。」
"So are you going to work overtime again today?" "Well actually, I was kind of hoping to get off work early today because I’ve been missing out on a lot of sleep."