I tried at night but was too tired, so now I get up a little earlier in the morning, make a cup of tea and write my journal. It helps set me up for the day.
At the moment only in the evening. But as I get back into the habit of using it I can see desire building to use it in the morning too. I am already finding myself being more mindful and awake in the morning, so I sort of start compiling to do lists and goals as I settle in to work. I think eventually I will develop a way to add that into my journal. I guess it depends a little on what your journal is for? If you're primarily using it for reflection rather than planning then evening only might be best for you?
I write both morning and evening. I set up my day and free write in the morning, and reflect in the evening. Sometimes I embellish it when I have breaks during the day.