It's a cycle. Eventually you figure out it's beneficial to have a consistent sleep schedule and eat healthier. I still stay up late sometimes and eat poorly too often, but less than I used to. The posted text is a better description of young adults.
Start naturally just going to sleep earlier and naturally waking up earlier.
Whilst it's not a 100% fullproof thing, it tends to help a lot because one just gets sleepy early and feels like going to sleep rather than trying to force it (which, IMHO, doesn't really work) in order to try to sleep enough hours given the time of the day one must wake up at in order to work.
I do this once in a while and just try to keep it going, though eventually some longer period of heavy stress gets me back to 3 cups of expresso a day and not sleeping enough hours to wake up rested most nights :/
If you only eat fast food, stuff your face on snacks everyday, never have meaningful social interactions outside the internet, sleep very little, don't exercise and only work all day without a hobby or engaging entertainment. Yes, you'll eventually become sad and tired all the time.
thats why east asian schooling is better. when you go into the workforce you go from 12 hrs a day for school to 8 hrs a day for work. Life gets easier.