Sleep is an extremely interesting phenomenon in which the mind withdraws almost completely from the usual realm of consciousness. It is distinguished by quiet wakefulness and a reduced ability to react to stimuli in which we become less aware of our surroundings. However, it is easier to reverse the situation than hibernation or coma. It is a function that has been extensively studied by many. After all, we wouldn't have developed a mechanism that forces us to spend a third of our lives sleeping unless sleep is good for us. What is the hard purpose? Over the years, many theories have been proposed as to why we need sleep. The simplest is that it saves energy. An individual's energy expenditure and demand is reduced during the day or night, like an animalistic instinct when it is less efficient in seeking food. This is also supported by the decrease in body temperature and calorie needs during 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 bodies, as they need time to recover and slow down. Additionally, sleep provides an opportunity for the brain's restorative functions where the body is given sufficient time to repair and rejuvenate itself. For example, sleep-deprived animals completely lose all their immune functions 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 wanes, a... middle of paper... a consolidation of memory." An Idiot's Guide to Dreaming. Web. June 19, 2011. .Kalat, James W. Introduction 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. scientific and reconsiderations.Cambridge, UK: Cambridge UP, 2003. Print.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. "Sleep, dreams and memory consolidation: the role of the stress hormone cortisol". 2011. .