I'm with George on this one. I like to finish off a day with the atomic module/query/code I'm working on working nicely (optimized or not; it needs to function). Somehow, in my mind, it's like closing the trailing bracket. I like to finish the day 'clean'.

Sure, this might cause the next day to be a slower start, but you have to balance this out against the peace of mind you get from leaving the office and having no worry about. That is worth a lot (and might cause me to sleep better and be more productive the next day?)

It's like doing the dinner dishes before going to bed so you don't have to wake up to them (or not enjoying the rest of the evening because you think you'll have to do them in the morning).

Like George, I'll often plan my next day ahead so that I know what I'm going to do to hit the ground running (Optimize the code I wrote last night, add input error correction, cough add comments or tighten up the format ...)

All these things aside. Do what works for you!