If we are on the same schedule as most others, the air is fresher, it is cooler, there is more open space available outside with fewer other people interrupting us for whatever reason, if we conduct our training in the early morning. Especially training of a solely fitness or meditative, self-cultivating nature, such as Hei Gung/ Breathing Skills...
Who wants to do that during evening sessions, with everyone around?
At best, it is distracting. At worst, it's wasting time that is more productively spent on interaction (Here, I'll be right with you...let me sit my Horse, practice Tit Sin Kyun, and train several thousand strokes of my favorite technique with footwork and without and...oops, you've got to go already?)...
Early morning energy work should be involved with internal development and building the air. Ppl training physically at 12-3AM are 'on their own' with that, ok...
Best,
Michael