Short term sleep deprivation isn't really harmful although it's still not good for your health.
But overtime if you become even more sleep deprived and don't get the required amount of sleep your body can start to shut down, and the sleep deprivation can also lead to other serious health conditions such as heart disease, coronary heart disease, heart attacks and strokes.
Your brain cells can also start to die off eventually although it can take awhile for them too and you risk getting into accidents that could be harmful or deadly because when you don't get enough sleep you're not alert as you would be when you're well rested.
This increases your chance of getting in car accidents or any other accidents when you're not alert and able to focus.