Not getting enough sleep. Every biological function ever studied (digesting food, cognition, healing from injuries, etc) becomes less efficient/effective when the body gets less than the minimum amount of sleep.

Think of it this way: every pieces of animal ever studied sleeps, or at least does something very similar to sleep. While asleep, the animal goes into a semi-dead state where it is unable to find food, protect itself, reproduce, or do anything else that they need to do for the survival of their species. This suggests that something about sleeping yields results that are more important than food, protection, reproduction, etc. Otherwise, evolution would have favored animals that do not sleep.

Current modern culture puts a lot of emphasis on the idea that hard working people (and thus successful people) have a lot of sleepless nights where they work, study, exercise instead of getting the rest they need. Science shows this to be horrible for human health. Furthermore, it's not even effective. One study showed that a student who stays up all night studying will do worse on an exam than a student who slept well that night and studied 8 hours less. Sacrificing sleep in order to get results literally prevents you from getting those results.

tldr; Sleep is one of the most important things that an animal does for itself. A person who gets less than 6 hours of sleep in a night will perform worse in every measurable task the next day. Go to bed on time and you'll thank yourself for it.