I used to work out n the morning without a lot of food.
Not a good idea. Took me a couple of hours to get back on track.
And I could literaly tell when my body "ran out of energy". (kinda like an engine running out of fuel)
Now I tend to get up a bit earlier and try to eat more, especially more FAT. it kinda keeps my body warm. (maybe just placebo, but still). rest a few minutes and then get into my warm up routine.