Sleep deprivation (DEP-rih-VA-shun) is a condition that occurs if you don't get enough sleep. Sleep deficiency is a broader concept. It occurs if you have one or more of the following:
                
You don't get enough sleep (sleep deprivation)
                
You sleep at the wrong time of day (that is, you're out of sync with your body's natural clock)
                
You don't sleep well or get all of the different types of sleep that your body needs
                
You have a sleep disorder that prevents you from getting enough sleep or causes poor quality sleep
                
This article focuses on sleep deficiency, unless otherwise noted.
                
Sleeping is a basic human need, like eating, drinking, and breathing. Like these other needs, sleeping is a vital part of the foundation for good health and well-being throughout your lifetime.
                
Sleep deficiency can lead to physical and mental health problems, injuries, loss of productivity, and even a greater risk of death.
                
Overview
                
To understand sleep deficiency, it helps to understand how sleep works and why it's important. The two basic types of sleep are rapid eye movement (REM) and non-REM.
                
Non-REM sleep includes what is commonly known as deep sleep or slow wave sleep. Dreaming typically occurs during REM sleep. Generally, non-REM and REM sleep occur in a regular pattern of 35 cycles each night.
                
Your ability to function and feel well while you're awake depends on whether you're getting enough total sleep and enough of each type of sleep. It also depends on whether you're sleeping at a time when your body is prepared and ready to sleep.
                
You have an internal "body clock" that controls when you're awake and when your body is ready for sleep. This clock typically follows a 24-hour repeating rhythm (called the circadian rhythm). The rhythm affects every cell, tissue, and organ in your body and how they work. (For more information, go to "What Makes You Sleep?")
                
If you aren't getting enough sleep, are sleeping at the wrong times, or have poor quality sleep, you'll likely feel very tired during the day. You may not feel refreshed and alert when you wake up.
                
Sleep deficiency can interfere with work, school, driving, and social functioning. You might have trouble learning, focusing, and reacting. Also, you might find it hard to judge other people's emotions and reactions. Sleep deficiency also can make you feel frustrated, cranky, or worried in social situations.
                
The signs and symptoms of sleep deficiency may differ between children and adults. Children who are sleep deficient might be overly active and have problems paying attention. They also might misbehave, and their school performance can suffer.
                
Outlook
                
Sleep deficiency is a common public health problem in the United States. People in all age groups report not getting enough sleep.
                
As part of a health survey for the Centers for Disease Control and Prevention, about 719 percent of adults in the United States reported not getting enough rest or sleep every day. 
                
Nearly 40 percent of adults report falling asleep during the day without meaning to at least once a month. Also, an estimated 50 to 70 million Americans have chronic (ongoing) sleep disorders.
                
Sleep deficiency is linked to many chronic health problems, including heart disease, kidney disease, high blood pressure, diabetes, stroke, obesity, and depression.
                
Sleep deficiency also is associated with an increased risk of injury in adults, teens, and children. For example, driver sleepiness (not related to alcohol) is responsible for serious car crash injuries and death. In the elderly, sleep deficiency might be linked to an increased risk of falls and broken bones.
                
In addition, sleep deficiency has played a role in human errors linked to tragic accidents, such as nuclear reactor meltdowns, grounding of large ships, and aviation accidents.
                
A common myth is that people can learn to get by on little sleep with no negative effects. However, research shows that getting enough quality sleep at the right times is vital for mental health, physical health, quality of life, and safety.