When deciding on using miles to upgrade on a red-eye, there are lots of things to consider:
* How full is the flight? I'd rather be in first than anywhere in a full 3 - 3 row.
* What am I doing the morning I land? If it's important and early, I will upgrade.
* How many miles do I have and what am planning to do with them? If the answer is "lots" and "don't know", I will probably upgrade.
There is really no right or wrong answer, just what works best for you.