Sleep is an extremely interesting phenomenon in which the mind drifts almost completely away from the usual realm of consciousness. This is distinguished from quiet wakefulness and a diminished ability to respond to stimuli where we become less aware of our surroundings. However, it is easier to reverse this situation than to be in hibernation or a coma. This is a feature that has been researched extensively by many people. After all, we wouldn't have developed a mechanism that would force us to spend a third of our lives sleeping if sleep didn't do us good. What's the use of hard? Over the years, many theories have been proposed as to why we need sleep. The simplest thing is that it saves energy. An individual's energy expenditure and demand is reduced during the day or night, due to animal instinct, when it is least efficient in foraging for food. This is also supported by the decrease in body temperature and caloric demand throughout sleep. For example, when NASA sent a robot to Mars, it was programmed to shut down at night so that exploration wouldn't waste energy. It's like our body, because it needs time to recover and slow down. Additionally, sleep provides an opportunity for restorative functions of the brain, where the body has ample time to repair and rejuvenate itself. For example, sleep-deprived animals completely lose all immune function and will soon die within a few weeks. Other findings have shown that many of the body's restorative functions, such as tissue repair, muscle growth, and growth hormone, occur primarily during sleep. When people are sleep deprived, inhibitory transmitters build up in the brain, interfering with attention and learning. Well-rested people will notice when their attention span wanes, a...... middle of paper...... Memory consolidation. " An Idiot's Guide to Dreaming. Web. June 19, 2011. .Kalat, James W. Introduction to Psychology. Belmont, CA: Thomson/Wadsworth, 2008. Print. Moss, Robert Dreaming True: How to Dream Your Future and Change Your life for the better. New York: Pocket, 2000. Print. Pace-Schott, Edward F. . Sleep and dreaming: scientific advances and reconsiderations. Cambridge, United Kingdom: Cambridge UP, 2003. Ratey, John J. and Albert M. Galaburda. A User's Guide to the Brain: Perception, Attention, and the Four Theaters of the Brain. New York: Pantheon, 2001. Print. stress hormone. » Learning and memory, June 19. 2011. .