Sleep stages

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STAGES OF SLEEP

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sleep is a state of perceptual disengagement from and unresponsiveness to the environment, marked by unique physiological and behavioral processes (Carskadon & Rechtschaffen, 2011).

During wakefulness, certain neurons fire in our brain, making us aroused and alert. However, during sleep, these neural circuits are inhibited, our muscles are completely relaxed, and our body becomes completely inactive (Schwartz & Roth, 2008).

There are five different stages of sleep including both REM (rapid eye movement) and NREM (non-rapid eye movement) sleep. The five stages make one sleep cycle which usually repeat every 90 to 110 minutes.

Stage 1 non-REM sleep marks the transition from wakefulness to sleep. This stage typically lasts less than 10 minutes and is marked by a slowing of your heartbeat, breathing, and eye movements , as well as the relaxation of your muscles.

Stage 2 non-REM sleep is a period of light sleep before you enter deeper sleep, lasts roughly 20 minutes. Stage two is characterized by further slowing of both the heartbeat and breathing, and the brain begins to produce bursts of rapid, rhythmic brain wave activity known as sleep spindles.

Formerly known as stages 3 and 4, stage 3 (N3) is the final stage of non-REM sleep. This is the deepest period of sleep and lasts 20 to 40 minutes. Your heartbeat and breathing slow to their lowest levels, and your muscles are so relaxed that it may be hard to awaken you.

REM sleep occurs 90 minutes after sleep onset, and is a much deeper sleep than any of the three stages of non-REM sleep. REM sleep is defined by rapid eye movements and an almost complete paralysis of the body, and a tendency to dream.

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The Circadian Rhythm

But how do our bodies know when to go back and forth between these states of sleep and wakefulness? Luckily, we have all an internal clock telling us when to do so. The circadian rhythm, our 24-hour clock, operates as our sleep/wake cycle.
This body clock is located in the suprachiasmatic nucleus (SCN), a pair of distinct groups of cells located in the hypothalamus (Dubokovich, 2007). It controls the production of melatonin, a hormone that makes you sleepy, by receiving information about incoming light from the eyes (Dubokovich, 2007).
When there is less light (like at night), the SCN tells the brain to make more melatonin so you get drowsy and are able to fall asleep. After a night’s rest, melatonin levels will drop, and you will be awake for the day.

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Measurement tools

Electroencephalogram (EEG)—measures and records the brainwave activity to identify sleep stages and detect seizure activity.

∘ Electrooculogram (EOG)—records eye movements. These movements are important for identifying the different sleep stages, especially the REM stage.

∘ Electromyogram (EMG)—records muscle activity (e.g., teeth grinding and face twitches; but also, limb movements using surface EMG monitoring of limb muscles, periodic or other). Chin EMG is necessary to differentiate REM from wakefulness, limb EMG can identify periodic limb movements during sleep (PLMS).

∘ Electrocardiogram (EKG)—records the heart rate and rhythm.

∘ Pulse oximetry—monitors the oxygen saturation (SO2).

∘ Respiratory monitor—measures the respiratory effort (thoracic and abdominal). It can be of several types, including impedance, inductance, strain gauges, etc.

∘ Capnography—measures and graphically displays the inhaled and exhaled CO2 concentrations at the airway opening.

∘ Transcutaneous monitors—measure the diffusion of O2 and CO2 through the skin.

∘ Microphone—continuously records the snoring volume and kind.

∘ Video camera—continuously records video. It is useful to identify the body motion and position.

∘ Thermometer—records the core body temperature and its changes.

∘ Light intensity tolerance test—determines the influence of light intensity on sleep.

∘ Nocturnal penile tumescence test—is used to identify physiological erectile dysfunctions.

∘ Nasal and oral airflow sensor—records the airflow and the breathing rate.

∘ Blood pressure monitor—measures the blood pressure and its changes.