Short term lack of sleep isn't all that serious and usually only causes irritability, slower reaction times, confusion, loss of concentration and high blood pressure can all happen due to lack of sleep.
If the lack of sleep continues then brain cells can die, and lack of sleep over a longer period of time can lead to serious health and medical conditions such as heart disease, heart attack, strokes, diabetes and lack of sleep can also cause diabetes.
Our bodies need the sleep to repair itself and recharge our batteries and without enough sleep to recharge our batteries we're drained during the next day and have no energy to do anything.