Personally, I much prefer taking stock in the early afternoon. That's the time when I tend to be feeling low on energy and motivation, and need an extra bit of inspiration to keep me going. And if I decide/discover that I need to make some change in my life, I can do it that day.

What happens when you start reflecting on the day (or planning the next) late at night? You get excited, or you remember something you forgot to do -- either way, that makes it hard to sleep. And the next day, all the enthusiasm is lost.