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The Necessity of Sleep Sleep researcher William Dement once claimed that the national sleep debt is a greater threat to the United States than the national monetary debt. Most people do not get an adequate amount of shut-eye. In a 2009 poll, 20 percent of Americans reported getting less than six hours of sleep per night. Why do we need sleep? The answer seems obvious. Without it, we become walking zombies, propelled through the day by caffeine and weary determination alone. Though no definitive answers exist concerning its precise purpose, sleep is believed to help restore body tissues and assist in the growth process. Circadian Rhythm It’s Saturday morning—your one day to sleep in. To your dismay, you wake at the time your alarm normally goes off and have trouble getting back to sleep. That happens because your body is acting in accordance with its biological clock, or circadian rhythm (from the Latin  circa , “about,” and  dies , “day”). A region of your brain called the hypothalamus

muscle , contractile  tissue  found in animals, the function of which is to produce  motion . Movement, the intricate cooperation of muscle and  nerve  fibres, is the means by which an organism interacts with its  environment . The innervation of muscle cells, or fibres, permits an  animal  to carry out the normal activities of life. An organism must move to find food or, if it is sedentary, must have the means to bring food to itself. An animal must be able to move nutrients and fluids through its body, and it must be able to react to external or internal stimuli. Muscle cells fuel their actions by converting chemical  energy  in the form of  adenosine triphosphate  (ATP), which is derived from the  metabolism  of food, into  mechanical energy .   Muscle is contractile tissue grouped into coordinated systems for greater  efficiency . In humans the muscle systems are classified by gross appearance and location of cells. The three types of muscles are  striated  (or skeletal), cardiac,

  digestive system disease , any of the diseases that affect the human   digestive tract . Such disorders may affect the esophagus, stomach, small intestine,   large intestine   (colon), pancreas, liver, or biliary tract. A prevalent disorder of the digestive system is   gastroesophageal reflux disease   (i.e., the passage of gastric contents into the esophagus), which causes heartburn on a regular basis in some individuals.   Cirrhosis   of the liver primarily results from excessive   alcohol consumption , but it may also develop after infection with the   hepatitis   C virus. Other common diseases of the digestive system include   peptic ulcers ,   colorectal cancer , and   gallstones . Many disorders of the digestive system can be prevented by a diet low in fats and high in fruits and vegetables, limited alcohol   consumption , and periodic medical examinations. This article discusses the common infections, inflammations, ulcers, and cancers that affect each organ of the digestive t

  Respiratory system The human gas-exchanging organ, the  lung , is located in the thorax, where its delicate tissues are protected by the bony and muscular thoracic cage. The lung provides the tissues of the  human body  with a continuous flow of oxygen and clears the blood of the gaseous waste product,  carbon dioxide . Atmospheric  air  is pumped in and out regularly through a system of pipes, called conducting airways, which join the gas-exchange region with the outside of the body. The airways can be divided into upper and lower  airway  systems. The transition between the two systems is located where the pathways of the respiratory and  digestive systems  cross, just at the top of the  larynx . human nasal cavity The upper airway system  comprises  the nose and the paranasal cavities (or  sinuses ), the  pharynx  (or throat), and partly also the  oral cavity , since it may be used for breathing. The lower airway system consists of the larynx, the  trachea , the stem bronchi, and