What Is Sleep Architecture?

Sleep architecture is a reference to how an individual cycles through the stages of sleep. This is often measured during a sleep study, or polysomnogram (PSG). The percentage of time spent in each stage of sleep is calculated and reported to determine if a person is getting enough rapid-eye-movement (REM) sleep. Typically, the percentage is calculated either as a percentage of "time in bed" (TIB) or as a percentage of "total sleep time" (TST).

If a person has sleep apnea or other sleeping disorders causing frequent arousals, the sleep architecture will be affected. A lack of sufficient REM sleep can cause depression, mental confusion, high blood pressure, aggression, hand tremors, and a decrease in cognitive skills.

The amount of time spent in each stage of sleep varies by an individual's age, ethnicity, and genetic profile. There are general guidelines as to "normal" amounts of each sleep stage, however, so a person who only spends 5% of their total sleep time in REM sleep would be identified as having a problem with their sleep architecture.

The amount of time spent in Stage 1 sleep is minimal. This stage of sleep is extremely light, and if a person is woken they will often state they were not sleeping. This stage of sleep may be described as "drifting off," and electroencephalogram tracings will show a decrease in brain activity as a person transitions from wakefulness to Stage 1 sleep.

A unique phenomenon occurs during this stage of sleep: a person may feel as if they are suddenly falling and jerk awake. This is called hypnic myoclonia, or a hypnic jerk. These incidents are more common in children, and are also noted more frequently in people who have irregular sleep schedules.

As the body enters Stage 2 sleep, the heart rate slows and the body temperature begins to drop. The muscles may alternate between having tone and being completely at rest during this sleep stage. This is still classified as light sleep, and a person will wake fairly easily from this sleep stage. They will feel as if they were asleep if wakened.

The EEG tracings will show peaks and valleys, or "waves." The alpha waves of Stage 1 sleep show interruptions by spikes called "sleep spindles" and "K-Complexes." Theta brain wave activity is noted. This stage will be noted as "N2" or "NREM 2" sleep on a sleep study report.

Sleep spindles are brief bursts of brain activity, often followed by muscle twitching. The purpose of sleep spindles appears to be keeping the sleeper in a tranquil state: the spike in activity is generated between the thalamus and the cortex. One theory proposes that the muscle twitches allow a very young child's brain to learn about which nerves control specific muscles when the child is asleep. The sleep spindles allow an adult to sleep through a certain level of noise that would wake the sleeper in Stage 1 sleep. Interestingly, people with schizophrenia lack the normal pattern of fast and slow sleep spindles in sleep.

K-complexes engage the cortex and cerebral cortex, and are thought to be involved with processing of information during sleep. The complex often precedes a sleep spindle and shows a high voltage spike on an EEG. The complexes are spontaneous,but may be induced by touching a sleeping person in Stage 2 sleep.

Stage 3 and Stage 4 sleep are classified as "slow wave sleep" (SWS), characterized by delta waves on an EEG tracing. These stages used to be classified as different sleep stages, but are now considered a single sleep stage. This stage of sleep will be noted as "N3" or "NREM 3" sleep on a sleep study report.

In this phase of sleep, an individual is unresponsive to external stimuli. Noise, touch, or temperature changes will generally not wake the sleeper. This is the stage of "deep sleep" where muscle tissue regenerates, the immune system strengthens itself, and bone tissue grows.

Nighttime bed wetting occurs in this sleep stage, along with sleepwalking, night terrors, and talking while asleep. These sleep disorders are called parasomnias, and the majority of these problems decrease with age.

The amount of time spent in deep sleep decreases with age, and the elderly may spend little time (or even no time) in Stage 3/4 sleep.

REM sleep starts with the onset of high-frequency waves, similar in nature to the waves seen in people who are awake. The heart rate increases and the eyes move rapidly from side to side. This is the stage of sleep where dreaming occurs, and the brain is in an extremely active stage.The muscles are paralyzed in REM sleep, however, and a person cannot move while in this sleep state. This sleep stage is also called "paradoxical sleep," as the brain is highly active, but the body is paralyzed.

REM sleep is vital to the development of the brain. Very young infants spend as much as 50% of their total sleep time in REM sleep, and the amount of time spent in REM sleep decreases as a person ages. Adults spend approximately 20% of their total sleep time in REM sleep.

A lack of appropriate REM sleep will impair memory, cognition, and may cause attention deficit disorder in children.

Sleep architecture is measured in children when they are sent for a sleep study. There are many sleeping disorders that may affect a child's sleep cycles—any problem that causes frequent arousals will disturb the normal distribution of sleep stages. Problems common to children include:

• Periodic leg movements
• Sleep-disordered breathing
• Obstructive sleep apnea
• Central sleep apnea
• Parasomnias, such as sleep-walking or night terrors

Often, a sleep study will demonstrate a problem with sleep architecture and will also reveal the cause behind the disturbed sleep. In the case of sleep-disordered breathing, a physician will determine whether a child has apnea, and what type of apnea is present. In the case of obstructive apnea, a common course of action is to remove a child's tonsils and adenoids, as enlarged tonsils and adenoids are the single most common reason for obstructive apnea in children.

Each case must be evaluated by a physician and the appropriate therapy initiated - this may mean a tonsillectomy/adenoidectomy, evaluation by a neurologist, cardiologist, ear nose and throat doctor, or other specialist as indicated by the study. Once the underlying sleep disorder is treated, sleep architecture often improves dramatically.

Adults spend less time in REM sleep than infants, but a significant percentage of the night is still spent in this stage. Adults typically spent 20-25% of their total sleep time in REM sleep, with the other 75-80% of the time distributed among the other NREM sleep stages.

As adults age, less time is spent in deep sleep. Stage 4 sleep may entirely disappear in the elderly. As a person ages, sleep becomes more fragmented and the circadian rhythm becomes disturbed, causing insomnia and nighttime waking. Parasomnias like restless leg syndrome become more common. The reduced quality (and quantity) of sleep at night leads to an increase of daytime sleeping, which can further compound the problems of getting sufficient rest at night.

Timothy is a bright and slender seven year old boy, who experiences difficulty with bed wetting, occasional snoring, and mouth breathing. He is often moody at the end of the day and is a very "active" sleeper. His teacher at school reports he is often daydreaming, and has difficulty maintaining focus.

His pediatrician ordered a sleep study to determine if a cause could be found for his tiredness, mouth breathing, bed wetting, and moodiness. The sleep study was extremely helpful in identifying sleep related disorders, including interrupted sleep architecture, in Timothy. His sleep architecture is located in the table below:

Currently, Timothy is only experiencing less than half of the normal quantity of REM sleep expected in a child of his age. In addition, there is no Stage 4 sleep noted, though a significant amount of Stage 3 sleep could be observed.

Timothy's sleep study noted the presence of periodic leg movements, sleep-disordered breathing with obstructive sleep apnea and central apnea events, and frequent arousals due to oxygen desaturations (lowest oxygen level recorded was 83%).

Timothy was referred for evaluation by an otolaryngologist to determine if removing his tonsils and adenoids would be indicated, as the obstructive apnea is the main cause of his nighttime desaturations. Referrals to a cardiologist and a neurologist were also indicated due to the presence of central apnea associated with desaturation.