Introduction:

Sleep apnea is a disorder in which a person stops breathing during the night, perhaps hundreds of times, usually for 10 seconds or longer, sometimes for as long as a minute. These gaps in breathing are called apneas. The word apnea means absence of breath.

Sleep apnea is usually accompanied by snoring. People might not even know they have the condition. It inevitably causes daytime sleepiness.

Sleep apnea is grouped into three categories:

• Obstructive
• Central
• Mixed

Another, less severe form of obstructed breathing, is called upper airway resistance syndrome (UARS).

Obstructive Sleep Apnea

Obstructive sleep apnea (OSA) occurs when tissues in the upper throat collapse at different times during sleep, thereby blocking the passage of air. In general, OSA occurs as follows:

• On its way to the lungs, air passes through the nose, mouth, and throat (the upper airway).
• Under normal conditions, the back of the throat is soft and tends to collapse inward as a person breathes.
• Dilator (widening) muscles work against this collapse to keep the airway open. Interference or abnormalities in this process cause air turbulence.
• If the tissues at the back of the throat collapse and become momentarily blocked, apnea occurs. Breath is temporarily stopped. In most cases the person is unaware of it, although sometimes they awaken and gasp for breath.
• In some cases, the interference is incomplete (called obstructive hypopnea) and causes continuous but slow and shallow breathing. In response, the throat vibrates and makes the sound of snoring. Snoring can occur whether a person breathes through the mouth or the nose. (Snoring also occurs without sleep apnea.)
• Apnea decreases the amount of oxygen in the blood, and eventually this lack of oxygen triggers the lungs to suck in air.
• At this point, the patient may make a gasping or snorting sound but does not usually fully wake up.

Obstructive sleep apnea is defined as five or more episodes of apnea or hypopnea per hour of sleep in individuals who have excessive daytime sleepiness. Patients with 15 or more episodes of apnea or hypopnea per hour of sleep are considered to have moderate-to-severe sleep apnea.

Central Sleep Apnea

Central sleep apnea is much less common. It is caused by some problem in the central nervous system, most likely a failure of the brain to signal the airway muscles to breathe. In such cases, oxygen levels drop abruptly and usually the sleeper wakes with a start. Often people with central sleep apnea recall waking up. They generally experience less sleepiness during the day than people with obstructive sleep apnea. Heart disease, and in particular heart failure, is the most common cause of central sleep apnea.

Mixed Apnea

Mixed apnea is the term used when central and obstructive sleep apneas occur together.

Upper Airway Resistance Syndrome (UARS)

Upper airway resistance syndrome (UARS) is a condition in which patients snore, wake frequently during the night, and have excessive daytime sleepiness. However, UARS patients do not have the breathing abnormalities that characterize sleep apnea and they do not show a reduction in blood oxygen levels. Unlike apnea, UARS is more likely to occur in women than in men. Treatments are similar to those of sleep apnea. It is not known if UARS has any serious health complications.

Healthy Sleep

In sleep studies, subjects spend about one-third of their time asleep, suggesting that most people need about 8 hours of sleep each day. Individual adults differ in the amount of sleep they need to feel well rested, however. Infants may sleep up to 16 hours a day.

The daily cycle of sleeping and waking is called the circadian rhythm. It's commonly referred to as the biologic clock. Circadian means "about a day." Hundreds of bodily functions follow biologic clocks, but sleeping and waking comprise the most prominent circadian rhythm. The sleeping and waking cycle is about 24 hours. (People who are confined to windowless homes, with no clocks or other time cues, sleep and wake on a slightly longer cycle.) The 24-hour circadian rhythm typically adheres to the following factors:

• Humans are designed for daytime activity and nighttime rest.
• There is a natural peak in sleepiness at mid-day, the traditional siesta time.

In addition, daily rhythms mix with other factors that may interfere or change individual patterns:

• The firing of nerve cells in the brain may be faster or slower in different individuals.
• The monthly menstrual cycle in women can shift the pattern.
• Light signals coming through the eyes reset the circadian cycles each day, so changes in season or various exposures to light and dark can unsettle the pattern. The importance of sunlight as a cue for circadian rhythms is dramatized by the problems experienced by people who are totally blind. They commonly have trouble sleeping and other rhythm disruptions.

The Response in the Brain to Light Signals

The response to light signals in the brain is an important key factor in sleep:

• Light signals travel to a tiny cluster of nerves in the hypothalamus in the center of the brain, the body's master clock, which is called the supra chiasmatic nucleus or SCN.
• This nerve cluster takes its name from its location. It sits just above (supra) the optic chiasm, a major junction for nerves transmitting information about light from the eyes.
• The approach of dusk each day prompts the SCN to signal the nearby pineal gland to produce the hormone melatonin.
• Melatonin is thought to act as the body's time-setting hormone. The longer a person is in darkness the longer the duration of melatonin secretion. Secretion can be diminished by staying in bright light. Melatonin also appears to trigger the need to sleep.

Sleep Cycles

Sleep consists of two distinct states that alternate in cycles and reflect differing levels of brain nerve cell activity:

Non-Rapid Eye Movement Sleep (NonREM). NonREM sleep is also termed quiet sleep. NonREM is further subdivided into three stages of progression:

• Stage 1 (light sleep)
• Stage 2 (so-called true sleep)
• Stage 3 - 4 (deep "slow-wave," or delta sleep)

With each descending stage, awakening becomes more difficult. It is not known what governs NonREM sleep in the brain. A balance between certain hormones, particularly growth and stress hormones, may be important for deep sleep.

Rapid Eye-Movement Sleep (REM). REM sleep is active sleep. Most vivid dreams occur in REM sleep. REM-sleep brain activity is comparable to that in waking, but the muscles are virtually paralyzed, possibly preventing people from acting out their dreams. In fact, except for vital organs like lungs and heart, the only muscles not paralyzed during REM are the eye muscles. REM sleep may be critical for learning and for day-to-day mood regulation. When people are sleep-deprived, their brains must work harder than when they are well rested.

The REM/NREM Cycle. The cycle between quiet (NonREM) and active (REM) sleep generally follows this pattern:

• After about 90 minutes of NonREM sleep, eyes move rapidly behind closed lids, giving rise to REM sleep.
• As sleep progresses the NonREM/REM cycle repeats.
• With each cycle, NonREM sleep becomes progressively lighter, and REM sleep becomes progressively longer, lasting from a few minutes early in sleep to perhaps an hour at the end of the sleep episode.