Polysomnography (PSG)

Polysomnography (PSG), a type of sleep study, is a multi-parametric test used in the study of sleep and as a diagnostic tool in sleep medicine.

The components of a polysomnogram include the following:

  • Electromyography (EMG), specifically the submentalis muscle (bruxism) and anterior tibialis (restless legs)
  • Electro-occulogram (EOG), to monitor for rapid eye movements
  • Electrocardiogram (ECG), to monitor cardiac function
  • Airflow pressure and temperature (using nasal pressure transducer and thermistor)
  • Respiratory effort (using chest and abdominal belts)
  • Digital pulse oximetry (using finger probe)

During PSG, characteristic EEG, EMG, and EOG amplitudes and waveforms are scored in 30 second epochs.

Polysomnography Fig. 1

Normal, non-pathological sleep is characterized by a sleep latency (from lights out to Stage 1 sleep) of <30 minutes and only brief arousals during sleep (<10 seconds). Any nighttime awakenings are also brief, cognitively and affectively neutral, and the individual is able to quickly return back to sleep. The recommended amount of sleep varies by age, but declines with age.[1] Ultimately, the required sleep time is individualized – it is the amount of sleep required for a person to feel rested and restored the next day. Normal sleep is also characterized by the absence of daytime sleepiness.

Recommended Sleep Time

Adapted from: Hirshkowitz M, Whiton K, Albert SM, et al. National Sleep Foundation's sleep time duration recommendations: methodology and results summary. Sleep Health. 2015;1(1):40-43
Age Range Recommended Hours of Sleep
Newborn 0-3 months old 14-17 hours
Infant 4-11 months old 12-15 hours
Toddler 1-2 years old 11-14 hours
Preschool 3-5 years old 10-13 hours
School-age 6-13 years old 9-11 hours
Teen 14-17 years old 8-10 hours
Young Adult 18-25 years old 7-9 hours
Adult 26-64 years old 7-9 hours
Older Adult 65 or more years old 7-8 hours

Rapid Eye Movement (REM) sleep has two components: a parasympathetically-driven tonic component, and a sympathetically-driven phasic component. Phasic REM sleep is characterized by skeletal muscle twitches, increased heart rate variability, pupillary dilation, and increased respiratory rate. Tonic REM is characterized by EMG suppression (i.e. - no more muscle twitches), a high arousal threshold (much more difficult to wake up the individual), elevated brain temperature, poikilothermia (loss of thermoregulation), and penile tumescence (spontaneous erection).

All antidepressants (except for trazodone and bupropion) increase REM latency and suppress REM sleep. Atypical antipsychotic medications also generally suppress REM sleep, while increasing amounts of stage N3.

REM Latency

REM latency is the time from when someone falls asleep to the first onset of REM sleep. REM sleep occurs every 90 to 120 minutes throughout the night. Thus, changes in REM sleep latency are considered a markers for sleep-related disorders. Short REM latency (i.e. - faster time to REM onset) can be due to:

Adult Brain Waves (EEG) Fig. 1


The mnemonic BAT-D (Think of a bat!) can be used to remember the order of brain waves from wakefulness → deep sleep. Also, using the Greek letters, you can correspond the symbols to the lower bounds Hz of each wave.
  • B - Beta (β) [β looks like the numbers 1 and 3 mashed up – 13 Hz (lower bound)]
  • A - Alpha (α) [α looks like half of an 8 – 8 Hz (lower bound)]
  • T - Theta (Θ) [Θ looks partially like a 4 – 4 Hz (lower bound)]
  • D - Delta (δ)

Sleep consists of two physiological states: non-rapid eye movement sleep (known as NREM) and rapid eye movement sleep (also known as REM):

  1. NREM sleep can be broken down into three stages: N1, N2, and N3 (or four stages based on: Stages 1, 2, 3/4).
  2. REM sleep is a different state of sleep characterized by high levels of brain activity and physiological activity similar to an awake state. REM sleep usually develops about 90 minutes after sleep begins. REM sleep on a polysomnograph appears the same as the awake stage. Therefore, REM cannot be identified by EEG alone, you need an EMG (and see absence of muscle activity) to confirm the presence of REM

Sleep Stages

Stage of Sleep % Total Sleep Time Description EEG Eye Movements Muscle Tone
Awake - Wakeful state Alpha waves (high frequency, 8 to 12 Hz) Eyes move and blink High voluntary tonic activity
Stage N1 (Stage 1) 5% Also known as “light sleep.” It is the transition from wakefulness → sleep (and vice versa). Patients may often be in this stage, and think they are “not sleeping.” Increased light quality sleep indicates sleep disruption. Alpha waves (< 50% total) mixed with Theta waves Slow eye movements Decreasing levels of high tonic activity
Stage N2 (Stage 2) 50% Most of the night is spent in this stage! Sleep spindles and K-complexes emerge during this stage. They are thought to be the brain's way of evaluating potential threats (i.e. - external stimuli) while sleeping and to dampen arousals if the threats are not real. Theta waves, and sigma waves (also known as sleep spindles, 11-16 Hz) with K-complexes (negative sharp wave followed by positive slow waves, 12-14 Hz) None Low tonic activity
Stage N3 (Stage 3 and 4) 10-20% “Deepest” stage and hardest to awaken. Associated with sleep inertia when awoken in this stage. It is also the most restorative sleep. This is homeostatic sleep (reduced BP, HR, cardiac output, RR). growth hormone is released. Delta waves (low frequency, <2 Hz, with high voltage, >75 μV). None Low tonic activity
Rapid Eye Movement (REM) 25% A “paradoxical state” that resembles awake state, except there is muscle paralysis. Arousal and increased variability of autonomic state (BP, HR). Increased brain temperature, cerebral glucose metabolism and cerebral blood flow. Release of acetylcholine in the cortex is highest during waking and REM sleep. Irregular, low-voltage, fast waves, and mixed frequency activity, including: saw-tooth waves, theta activity, and slow alpha activity. Rapid eye movement Almost total muscle paralysis (during tonic phase of REM). There can be very brief movement during phasic periods.

Stages of Sleep with Polysomnograph Recordings Fig. 1

  • Stage 3 sleep is reduced by benzodiazepines
  • Stage 3 sleep is generally not affected by non-benzodiazepine receptor agonists (e.g. - Zopiclone)[2]
  • Individuals with depression have decreased sleep efficiency, decreased slow-wave sleep (meaning decreased stage 3 and stage 4 sleep time), reduced REM latency (the elapsed time from sleep onset until the beginning of the first REM period), and increased REM intensity.[3]

Insomnia is more prevalent in late life and sleep problems in the elderly are often mistakenly considered a normal part of aging. Sleep disturbance in older adults is associated with cognitive impairment, poor concentration, and decreased day-to-day functional performance. One of the challenges in identifying sleep disorders in older adults is that sleep architecture naturally changes significantly in healthy older adults (figure 4). In general with older adults:

  • Initiation of sleep is harder
  • Total sleep time (TST) and sleep efficiency are reduced
  • Restorative sleep (i.e. - delta wave or slow wave sleep) decreases
  • There is greater fragmentation of sleep, and there is generally longer time spent in bed awake
  • Natural age-related physiologic changes also change the circadian rhythm, and can cause older adults to retire to bed earlier and to wake up earlier[4]. Combined, all of these changes can result in less satisfactory sleep and less TST.[5]
  • REM sleep is usually preserved with age
  • Sleep latency decreases, making older adults more somnolent compared to younger adults

Hypnogram Comparing Sleep Across the Lifespan Fig. 4

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