In this space, we talk about Icelandic words for which there is no ready English equivalent, or just sound better than their English counterparts.
The word of this issue is svefngalsi. Svefngalsi is the kind of giddy hyperness you get from a lack of sleep, and is constructed from “svefn” (sleep, the noun) and “galsi” (exuberance). The closest English equivalents are “punch drunk” and “slap happy,” although these do not necessarily refer to the same things svefngalsi does.
Used in a sentence: “Hef verið á skrifstofunni í alla nótt og er nú með svefngalsa.” (“I have been at the office all night and now I have svefngalsi.”)