Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Coma is defined as a state of unconsciousness lasting more than six hours, in which a person cannot be awakened, fails to respond normally to painful stimuli, light, or sound; lacks a normal sleep-wake cycle and does not initiate voluntary actions. Although a coma patient may appear to be awake, they are unable to consciously feel, speak, hear, or move. Learn more about coma, its causes, symptoms and treatment in the following pages.