Consistent sleep deprivation will lead a person to succumb to certain chronic illnesses such as heart disease and diabetes mellitus. It is to be noted that a person who always fails to sleep at least eight hours per day will make one's immune system to eventually break down. Sleep is a vital activity wherein the body replenishes its' energy and builds up its' immune system. And if one's immune system is not effective enough in defending the body from the invasion of deadly microorganisms, then illness sets in.