Throughout the night, your body goes through two distinct stages of sleep: REM and non-REM. REM sleep is when the body reaches daytime levels, and during this stage, you’re prone to dreaming. Non-REM sleep slows down most body functions and reduces brain activity. You spend about half of the night in the “N2” stage, during which scientists believe you store long-term memories. The “N1” phase is when you’re most alert, but your brain isn’t.

The body and mind need adequate amounts of sleep for optimal performance. Without sleep, you can experience increased stress and concentration, lower energy levels, and an increased risk of sickness. Sleep also allows the brain to regenerate cells, replenish energy stores, and retain memory. The recommended amount of sleep is seven to eight hours. If possible, aim for over seven and a half hours. Sleep is essential for our health and well-being. Achieving this goal will make you more alert, productive, and happier in the long run.

While you can’t help but sleep, you should seek medical attention for any disorder that disturbs your sleep. Many of these conditions are common and treatable, including insomnia and sleep apnea. Some medications can contribute to insomnia, and your doctor may refer you to a specialist. For more serious cases, they may schedule a sleep study. Sleep disorders fall into six general categories and dozens of subtypes. A sleep study can help determine which one you have.

The three different stages of sleep involve several distinct physical changes. During non-REM sleep, your body temperature drops, heart rate decreases, and your eye movements cease. In non-REM sleep, your brain remains active, but your muscles and heart rate slow down. Your breathing and heart rate also slow down. Your brain also stops using the chemical substances that keep you awake, making it hard to wake up in this stage. However, the most important part of sleep is resting in a comfortable position.

While most adults need seven to nine hours of sleep each night, many suffer from too little sleep and are unaware of it. In fact, one in three American adults reports sleeping fewer than seven hours per night, while two-thirds of high school students report falling into REM sleep. Too little sleep can cause many short-term and long-term problems, ranging from chronic fatigue and depression to accidents. Sleep disorders affect millions of Americans, and are considered a serious health concern. There are six general types of sleep disorders.

Insomnia is a condition that affects about 12 million people in the U.S. A lack of sleep impairs the brain’s ability to function properly, causing people to be unable to get restful sleep. It can also lead to other health conditions, including high blood pressure, a shorter life span, and increased accident risk. People with insomnia may experience restless leg syndrome, a condition in which leg movement is constantly involuntarily during the day and triggered at night.