For certain types of academic work (e.g., writing a book) the mental cost of initially loading all the relevant material into working memory takes a very long time, so once I've paid this upfront cost, I tend to keep working as much as physically possible to amortize the upfront costs.

If I stop working at 5pm like salaried ppl (or say 8pm like startup ppl), then the next day I'll have to spend another 2--3 hours to get back to the same mental state. I rather continue push on and ship something tonight (with some damaging health effects) than pay the upfront costs again tomorrow.