Stress weakens the immune system and has other serious effects on the brain and body such as cardiovascular problems, and mental illness such as depression.
Stress also kills brain cells and lowers your learning abilities or capabilities. Specifically, long periods of stress destroys brain cells in the hippocampus and learning and memory capabilities can be greatly reduced. Another factor is that the brain absorbs information better when it is relaxed, in a positive state of mind, and receptive to the information.
A tactic that is sometimes used, to break a person down psychologically and physically, is to induce as much stress as possible for a long period of time.
The key is the immune system. It can be weakened to leave the person vulnerable to serious illness like cancer or it can be strengthened to fight off powerful invaders and even cancer it’s self.
Acid-Base disorders, caused by high levels of stress and sleep deprivation, leads to osteoporosis or bone loss.
Cushing’s Syndrome, caused by high levels of stress and sleep deprivation, leads to muscle loss or muscle wasting.
Sleep deprivation prevents the growth hormone, which builds muscle and bone, from being released in the sleep cycle.