"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."
Remember how 不足 is a physical shortage? Well, now you have a physical shortage of sleep. That's a lack of sleep.
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.