Our bodies operate on a cycle run by an internal body clock, which closely mirrors the length of a full day. This 24-hour cycle is called a circadian rhythm. Circadian means “around the day” in Latin. Most of us feel alert when it’s light outside and experience a need to rest when it’s dark. But in circadian rhythm disorders, the body’s internal clock is out of sync with day and night, causing a disruption in normal waking and sleeping times.

About 90 million Americans suffer from snoring during sleep. While half of these people are “simple snorers,” the other half may have a serious sleep disorder called obstructive sleep apnea (OSA). While OSA will almost always lead to loud and frequent snoring, snoring does not always indicate OSA. Treatment of OSA often involves continuous positive airway pressure (CPAP). CPAP is a blower connected by a tube to a mask that fits over the mouth or nose, blowing air so that continuous pressure in the airway is maintained. If you or your partner experiences any symptoms of snoring or sleep apnea, consult a doctor.

Insomnia literally means “sleeplessness” and occurs when you have difficulty falling asleep or staying asleep. Insomnia may be temporary or it may be recurring— what doctors call “chronic.” Even during sleep, the brain is still active, moving through the stages of what is known as the sleep cycle. The sleep cycle has five recognized stages, and a full cycle lasts about 90 minutes. During the course of a typical night, a person goes through several sleep cycles. If you suffer from insomnia, you are not alone. By some estimates, one out of every four Americans deals with temporary insomnia, and one in 10 suffers from chronic insomnia.

Narcolepsy is a disorder of the central nervous system in which abnormal brain chemistry disrupts the normal wake/sleep cycle. People with narcolepsy often feel excessively sleepy during waking hours. In fact, they usually feel strong urges to sleep several times a day. If the urge becomes overwhelming, they can stay asleep for a few seconds or a few minutes. In rare instances, sleep attacks can last an hour or longer. Sudden bouts of sleep can occur at any time and in just about any situation. Narcolepsy occurs in about one out of every 2,000 Americans with the first symptoms usually appearing in young adults between the ages of 15 and 30.

The term “parasomnia” refers to all the abnormal things that can happen to people while they sleep, apart from sleep apnea. Some examples are sleep-related eating disorder, sleepwalking, nightmares, sleep paralysis, and REM sleep behavior disorder. Parasomnias can have negative effects on people during the daytime, including sleepiness. Parasomnias affect approximately 10 percent of Americans. They occur in people of all ages, but are more common in children.

If your doctor rules out sleep apnea, narcolepsy and other factors that could cause exhaustion (such as mental disorders or certain medications), he or she may consider a diagnosis of excessive daytime sleepiness or insufficient sleep. The best way to address this disorder is to look for its underlying cause. Your doctor may also suggest that you take naps throughout the day when possible, avoid alcohol and consider altering any prescriptions that trigger drowsiness.

Restless leg syndrome (RLS) is a commonly experienced neurological disorder. It is an uncomfortable feeling deep in the legs that can happen when sitting or lying down. The symptoms occur most often during the evening and night. The unpleasant sensations in one, or usually both, legs can range from mild to severe. RLS disrupts sleep and causes daytime fatigue. In addition to bedtime, a person may experience RLS when sitting still for long periods of time, such as during a plane flight or while sitting in a movie theater.

Sleep apnea occurs when breathing is briefly and repeatedly interrupted during sleep, marked by breathing pauses that last at least 10 seconds. It can cause fragmented sleep, low blood oxygen levels, and the combination of disturbed sleep and oxygen starvation may lead to hypertension, heart disease, and mood and memory problems. Treatments for sleep apnea focus on helping to keep the airway open through therapies or surgery.

Sleepwalking is also referred to as somnambulism. Sleepwalking is often seen among children, but can occur in people of all ages. By some estimates, up to 15 percent of Americans, or one in seven people, may experience sleepwalking at some point in their lives.