I have to do it as a part of my morning routine. If I say I'll do it later in the day, I almost never will. Also once the day is in full swing it can be even more difficult to calm my mind because first thing I still haven't thought of all the things I need to think about yet. I need to wake up a little earlier to leave time, otherwise it's difficult to not get bogged down by what I'm not doing yet. But if I woke up 10 mins earlier than usual, well that's time I could have been sleeping anyway!
Definitely morning – if you make this a habit you always have time to meditate. Also, morning meditations is a great way to become more focused and calm