Half the gym trains at 6am. What works before it, going by what clients do rather than what a supplement company says.
Section
If you are up at 5:15
Something small and fast: a banana, a slice of toast with honey, a small glass of milk or a yoghurt. Two or three hundred calories of mostly carbs, twenty to thirty minutes before. Coffee is fine.
If you cannot face food
Train fasted. For a 50-minute class it is not a problem for most people, as long as you eat properly afterwards. Water and a coffee.
The night before matters more
A normal dinner with carbs and protein does most of the fuelling for a 6am session. Skipping dinner to "save calories" is why the early class feels awful.
Afterwards, on the way to work
Protein plus carbs within an hour or two: eggs on toast, Greek yoghurt and fruit, a protein shake and a bagel, porridge with milk. This is where your nutrition plan targets come in.
Hydration
A pint of water when you wake up and a bottle during. Most people are dehydrated at 6am and blame the session.