Unfortunately, having a full-time job usually means that being able to write during the time period that is most effective or productive for you simply may not be possible. Because of that, you will need to determine what time you have available and then decide on where you can slice out a period of time that you can make available just for writing.
Many years ago when I started writing my first book, I was working 10-12 hours a day, and then coming home to a family with three pre-teen kids who were involved in a variety of activities. Needless to say, my free time was very limited and my ability to write when I really wanted to write was non-existent.
What I did was look at my daily schedule and try to find a way to squeeze in at least one or two hours of time where I could write with limited distractions and still feel productive. I tried getting up an hour or two earlier each day, but found that it took me too long to get started. I then tried to set aside an hour or two after the kids went to bed as my writing time. My wife at the time went to bed about the same time, so I had an hour or two each evening that was basically free of distractions, so that became my writing time. It took a couple of weeks to get accustomed to this new schedule, but once I did I found that I was getting a lot of writing done.
More recently, I started setting aside an hour each afternoon at work for my lunch break. I would bring my lunch and eat a quick meal, and then I would spend the rest of the time writing on my tablet. I used Dropbox to store a copy of my WIP so that I had easy access to it at work or at home and didn't have to worry about keeping things synchronized.
The important thing is to just find a time that works for you. It may not be your most productive time at first, but if you stick to it and make it a part of your daily routine, you will soon find that you are writing a lot more than ever imagined possible. I told myself that I was never going to HAVE the time, so it was up to me to MAKE the time!