The #1 Health Mistake
Voluntary circadian phase delay. A.K.A. staying up way too late.
People sometimes assume that sleep is just meant for the brain - mood, memory consolidation/retention, and focus - but it's clearly meant for the body as well. The downstream effects of going to bed and falling asleep at a reasonable time are numerous and include, building muscle mass, mobilizing fat stores, and optimizing immune function.
If you're not one for reading research, my post on sleep hygiene goes into greater detail regarding these effects.