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Polysomnogram (PSG) is a multi-channel ("poly") recording ("gram") during sleep ("somno"). A doctor may order a polysomnogram because the patient has a complaint such as daytime fatigue or sleepiness that may be from interrupted sleep. Typically, doctors order a polysomnogram to diagnose or rule out obstructive sleep apnea. Although the PSG can be done during the day or night, the vast majority of sleep studies are done at night, when most people sleep. Shift workers can be accommodated in some labs by having the test at other times.

For the standard test the patient comes to a sleep lab in the early evening, and over the next 1-2 hours is introduced to the setting and "wired up" so that multiple channels of data can be recorded when he/she falls asleep. The sleep lab may be in a hospital, a free-standing medical office, or in a hotel. A sleep technician is always in attendance and is responsible for attaching the electrodes to the patient and monitoring the patient during the study.


A polysomnogram usually records:

  • 2 channels for the electroencephalogram, or EEG. The EEG is crucial for determining a) IF the patient is sleeping or not, and b) what stage of sleep the patient is in (see below for stages). EEG may be recorded from multiple areas over the head, but for most PSGs two areas are sufficient: the back (occipital channel) and top (central channel).
  • 1 channel to measure air flow - this is done using a thermistor or pressure probe that fits inside the nostrils
  • 1 channel for chin movements - this is a recording of the chin 'electroymogram' or EMG, of muscle movements about the chin area; see Electromyography
  • 1 channel for leg movements - this is a recording of the electromyogram or EMG for the legs (usually one channel for both legs, though some labs will separate them into 2 separate channels); see Electromyography
  • 2 channels for eye movments, or 'electro-oculogram' - eye movements are crucial for determining the stage of sleep known as Rapid Eye Movement or REM sleep. REM sleep is when most of our dreaming takes place.
  • 1 channel for EKG or electrocardiogram - records heart rate and rhythm
  • 1 channel for oxygen saturation - this is done with a pulse oximeter that fits over a finger tip or the ear lobe
  • 1 channel for chest wall movement - using a belt that wraps around the chest
  • 1 channel for abdominal wall movement - using another belt that wraps around the upper abdomen
Pediatric polysomnography patient
Children's Hospital (Saint Louis), 2006

Thus, the typical polysomnogram has a minimum of 11 channels. (Note that this is different from the actual number of wires attached to the patient. For technical reasons, 2 wire attachments are actually required per individual recording channel in most cases.) The number of recorded channels can be more than 11 in certain situations. Some labs will measure air flow with both a thermistor and a pressure transducer (the latter considered more sensitive), so that the patient has 2 small probes in the nostrils, not one. Sometimes snoring will be recorded with a sound probe over the neck, though more commonly the sleep technician will just note snoring as "mild", "moderate" or "loud" or give a numerical estimate on "a scale of 1 to 10". Research labs and labs conducting special tests on selected patients (e.g., when nocturnal seizures are suspected) may also record additional data.

Wires for each channel of recorded data lead from the patient and converge into a central box, which in turn is connected to a computer system for recording, storing and displaying all the data. During sleep the computer monitor can display multiple channels continuously. In addition, most labs have a small video camera in the room so the technician can observe the patient visually from an adjacent room.

Despite all the attached wires and a new environment, most patients are able to sleep during the PSG. In fact, about the same number of patients state they slept 'as well or better' than at home, as state they slept not as well or poorly.

During the study, the technician observes sleep activity by looking at the video monitor and the computer screen that displays all the data second by second. In most labs the test is completed and the patient is discharged home by 7 a.m. After the test is completed a 'scorer' (usually not the sleep technician) analyzes the data by reviewing the study in 30 second 'epochs', looking for the following information:

  • Onset of sleep from time the lights were turned off; this is called 'sleep latency' and normally is less than 20 minutes. (Note that determining 'sleep' and 'awake' is based solely on the EEG. Patients sometimes feel they were awake when the EEG shows they were sleeping.)
  • Sleep efficiency: the number of minutes of sleep divided by the number of minutes in bed. Normal is approximately 85 to 90% or higher.
  • Sleep stages; these are based on 3 sources of data coming from 5 channels: EEG (2 channels usually), EOG (2) and chin EMG (1). From this information each 30-second epoch is scored as 'awake' or one of 5 sleep stages: 1, 2, 3, 4 and REM or Rapid Eye Movement sleep. Stages 1-4 are together called non-REM sleep. Non-REM sleep is distinguished from REM sleep, which is altogether different. Within non-REM sleep, stages 3 and 4 are called "slow wave" sleep because of the relatively wide brain waves compared to other stages; another name for stages 3 and 4 is 'deep sleep'. By contrast, stage 1 and 2 are 'light sleep.'. The figures show Stage 4 sleep and REM sleep; each figure is a 30-second epoch from an overnight PSG.
Stage 4 Sleep. EEG highlighted by red box.
Polysomnographic record of REM Sleep . EEG highlighted by red box. Eye movements highlighted by red line.

(The percentage of each sleep stage varies by age, with decreasing amounts of REM and deep sleep in older people. The majority of sleep at all ages (except infancy) is Stage 2. REM normally occupies about 20-25% of sleep time. Many factors besides age can affect both the amount and percentage of each sleep stage, including drugs (particularly anti-depressants and pain meds), alcohol taken before bed time, and sleep deprivation.)

  • Any breathing irregularities; mainly apneas and hypopneas. Apnea is a complete or near complete cessation of breathing for at least 10 seconds; hypopnea is a partial cessation of breathing for at least 10 seconds.
  • 'Arousals' are sudden shifts in brain wave activity. They may be caused by numerous factors, including breathing abnormalities, leg movements, environmental noises, etc. An abnormal number of arousals indicates 'interrupted sleep' and may explain a person's daytime symptoms of fatigue and/or sleepiness.
  • Cardiac rhythm abnormalities
  • Leg movements
  • Body position during sleep
  • Oxygen saturation during sleep

Once scored, the test recording and the scoring data are sent to the sleep medicine physician for interpretation. Ideally, interpretation is done in conjunction with the medical history, a complete list of drugs the patient is taking, and any other relevant information that might impact the study such as napping done before the test.

Once interpreted, the sleep physician writes a report which is sent to the referring physician, usually with specific recommendations based on the test results.

Example of summary report from overnight 'diagnostic' sleep study (PSG)

Mr. J-----, age 41, 5’8” tall, 265 lbs., came to the sleep lab to rule out obstructive sleep apnea. He complains of some snoring and daytime sleepiness. His score on the Epworth Sleepiness Scale is elevated at 15 (out of possible 24 points), affirming excessive daytime sleepiness (normal is <10/24).

This single-night diagnostic sleep study shows evidence for obstructive sleep apnea (OSA). For the full night his apnea+hypopnea index was elevated at 18.1 events/hr. (normal <5 events/hr; this is “moderate” OSA). While sleeping supine, his AHI was twice that, at 37.1 events/hr. He also had some oxygen desaturation; for 11% of sleep time his SaO2 was between 80% and 90%.

Results of this study indicate Mr. J---- would benefit from CPAP. To this end, I recommend that he return to the lab for a CPAP titration study. (Interpreted by Dr. M.)

'Split night' sleep studies

The above report mentions CPAP as treatment for obstructive sleep apnea. CPAP is continuous positive airway pressure, and is delivered via a tight fitting mask to the patient's nose or nose & mouth (some masks cover one, some both). CPAP is typically prescribed after the diagnosis of OSA is made from a sleep study (i.e., after a PSG test). To determine the correct amount of pressure, the right mask size, and also to make sure the patient is tolerant of this therapy, a 'CPAP titration study' is recommended. This is the same as a 'PSG', but with the addition of the mask applied, so the technician can increase the airway pressure inside the mask as needed, until all (or most all) of the patient's airway obstructions are eliminated.

The above report recommends Mr.J---- return for a CPAP titration study, which means return to the lab for a 2nd all night PSG (this one with the mask applied). Often, however, when a patient manifests OSA in the first 2 or 3 hours of the intial PSG, the technician will interrupt the study and apply the mask right then and there; the patient is literally woken up and fitted for a mask. The rest of the sleep study is then a 'CPAP titration.' When both the diagnostic PSG and a CPAP titration are done the same night, the entire study is called 'Split Night'.

The advantages of the split night study are: 1) the patient only has to come to the lab once, so it is less disruptive than coming two different nights; 2) it is 'half as expensive' to whomever is paying for the study. The disadvantages of a split night study are 1) less time to make a diagnosis of OSA (Medicare requires a minimum of 2 hours of diagnosis time before the mask can be applied); and 2) less time to assure an adequate CPAP titration. If the titration is begun with only a few hours of sleep left, the remaining time may not assure a proper CPAP titration, and the patient may still have to return to the lab.

Because of costs, more and more studies for 'sleep apnea' are attempted as split night when there is early evidence for OSA. Note that both types of study - with and without a CPAP mask - are still polysomnograms. When the CPAP mask is worn, however, the flow measurement lead in the patient's nose is removed, and a wire coming directly from the mask then measures air flow.

Example of summary report from a 'split night' sleep study (PSG)

Mr. B---, age 38, 6 ft. tall, 348 lbs., came to the Hospital Sleep Lab to diagnose or rule out obstructive sleep apnea. This polysomnogram consisted of overnight recording of left and right EOG, submental EMG, left and right anterior EMG, central and occipital EEG, EKG, airflow measurement, respiratory effort and pulse oximetry. The test was done without supplemental oxygen. His latency to sleep onset was slightly prolonged at 28.5 minutes. Sleep efficiency was normal at 89.3% (413.5 minutes sleep time out of 463 minutes in bed).

During the first 71 minutes of sleep Mr. B---- manifested 83 obstructive apneas, 3 central apneas, 1 mixed apnea and 28 hypopneas, for an elevated apnea+hyponea index (AHI) of 97 events/hr. (= “severe” OSA). His lowest SaO2 during the pre-CPAP period was 72%. CPAP was then applied at 5 cm H2O, and sequentially titrated to a final pressure of 17 cm H2O. At this pressure his AHI was 4 events/hr. and the low SaO2 had increased to 89%. This final titration level occurred while he was in REM sleep. Mask used was a Respironics Classic nasal (medium-size).

In summary, this split night study shows severe OSA in the pre-CPAP period, with definite improvement on high levels of CPAP. At 17 cm H2O his AHI was normal at 4 events/hr. and low SaO2 was 89%. Based on this split night study I recommend he start on nasal CPAP 17 cm H2O along with heated humidity. (Interpreted by Dr. M.)


Pressman MR. Primer of Polysomnogram Interpretation. Butterworth Heinemann, Boston (2002). ISBN 0-7506-9782-2
Berry RB. Sleep Medicine Pearls. Hanley & Belfus, Philadelphia (2003). ISBN 1-56053-490-7.
Bowman TJ. Review of Sleep Medicine. Butterworth Heinemann, Boston (2003). ISBN 0-7506-7392-3.
Kryger MH, Roth T, Dement WC. Principles and Practice of Sleep Medicine, 4th edition. Elsevier Saunders, Philadelphia (2005). ISBN 978-0-7216-0797-9.
Kushida CA, Littner MR, Morgenthaler TM, et al. Practice parameters for the indications for polysomnography and related procedures: An update for 2005. Sleep 2005;28:499-519.

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