Explore your journal writing at various times of the day to see what works best for you. Some people write when they first wake up in the morning. This way their thoughts have not been disrupted by anything yet. Some people will journal in the afternoon after lunch. Others are night writers who like to do reflective journaling to touch on things that happened throughout the day.
Personal journaling is a gift you give to yourself. Whatever time you choose, make sure that you carve out enough time for yourself to focus on your writing. I like to write at night; the nighttime is my time to unwind. However, if the mood strikes, I will journal anytime, anywhere.