Evening. As I like to document the day I spent. But a journal doesn't have to be a documentary. You can be brimming with all sorts of thoughts and ideas in the morning… opinions on different items etc which you can journal … a journal is to document your thoughts.
I always write in the evening to reflect on how I felt that day, what I have done, etc. Sometimes if I wake up not feeling motivated I will write when I get up but for me it's a new habit I'm trying to start to kick start my days.