Sleep is an altered state of consciousness characterized by relaxed skeletal muscles and lack of goal-directed behaviour. In humans and many other mammals, the characteristic posture is a horizontal repose, indicating a passive role towards the environment. Despite its similarity to daytime wakefulness, sleep is more difficult to define than other states of consciousness. The brain’s activity and response to sensory input is regulated by the sympathetic nervous system, which helps maintain body stillness.

In most cases, sleep is defined by the convergence of observational criteria that are deemed consistent with the state of sleep. These criteria include behavioral, sensory, motor, and physiological elements. However, some of these criteria are absent or varying among individuals, particularly during sleepwalking or wakefulness. Therefore, it’s important to use objective measures to distinguish between sleep and wakefulness.

Sleep is vital to your health and well-being. It helps your brain process information, reorganizes brain cells, and maintains emotional balance. It also produces antibodies and immune cells that fight infection and inflammation. This makes it even more important to get enough sleep, especially when you are stressed or sick. Research also suggests that adequate sleep helps your heart’s health. Getting less than seven hours of sleep a night can increase your risk of cardiovascular disease and other health problems.

Rapid eye movement (REM) sleep has a complex pattern of brain activity. REM sleep involves heightened activity in the brain’s thalamus, a region that is quiet during most stages of sleep. This area sends sensory information to the cortex, which translates it from short-term to long-term memory.

According to the American Sleep Association, approximately 40 million people in the United States suffer from a sleep disorder every year. Twenty million others experience occasional problems. These problems can interfere with work, driving, and other activities. In addition, they cost the health system approximately $16 billion annually. Fortunately, most of these disorders are treatable.

Getting enough sleep is important for everyone. However, the exact amount of sleep a person needs depends on his/her age and physical condition. People in their early 20s, for example, require fewer hours of sleep than older adults. This is because they may not feel fully rested even after a full night’s sleep. In addition to rest, sleep helps the body repair itself after a stressful day.

During a night’s sleep, the brain goes through three stages: REM, NREM, and N3 sleep. Each phase lasts about an hour and a half. As a result, the brain becomes less responsive to the outside world, and most body functions slow down. The first stage, or “N1,” occurs in the first third of the night. During this phase, you tend to have vivid dreams.

While the human body has many different sleep patterns, the two most common patterns occur during the first 90 minutes of sleep. Infants typically have six to seven sleep cycles throughout the day, while children may have a morning and an afternoon nap. The REM stage typically lasts between 10 minutes and an hour.