If you’re reading this, you’ve probably spent the night in a polysomnographic laboratory, linked up to machines that generate encyclopedic amounts of data on what’s going on in your brain and blood, lungs and limbs.

Neither you nor the doctor who is treating you will see more than a minuscule portion of this information. The sleep lab will distill the material’s 1,000 or so pages down to the basics. Depending on the lab,you may receive a paragraph, a one-page summary, or a six-page paper complete with graphs. Even in a condensed version, there is a lot of information, and it can be very confusing. “People do get lost in the numbers,” says Dr. Rochelle Goldberg, a sleep medicine specialist and previous president of the American Sleep Apnea Association. Not only are there numbers to learn, but there are also acronyms and vocabulary to learn.

Here is a guide on polysomnography to assist you acquire a better grasp on these numbers and alphabet soups, as well as a better understanding of what they quantify and imply. The following ASAA members contributed to this guide: Dr. Richard E. Waldhorn, a professor and clinician at Georgetown University and past association president; Goldberg; and registered polysomnographic technologist David Polaski. In addition, we drew from a report published in the May 1999 issue of Sleep by the American Academy of Sleep Medicine Task Force, chaired by Dr. W. Ward Flemons.


The total sleep time, or TST, is the first factor considered. This is an objective measure of the length of time you actually sleep throughout your research, based on readings from electrodes that record brain waves. This measurement frequently contradicts people’s subjective perceptions of how much they slept. They’ll feel as if they didn’t get a wink of sleep, yet the report says they slept for six hours. The electrodes are not deceiving.


The sleep efficiency is defined as the ratio of total sleep time to total recording time, or TRT. People who have severe difficulties initiating or maintaining sleep have worse sleep efficiency, which can be linked to a variety of diseases and disorders, including depression. The sleep onset latency is the number of minutes that pass between the time the light is turned off and the person falls asleep. Normally, falling asleep takes around 15 minutes. A much shorter onset latency–the proverbial “falling asleep as soon as your head hits the pillow”–may appear pleasant, but it actually indicates sleep deficiency. This could indicate a disease, or it could simply be a signal to calm down. Regardless of the obligations of work and family, as well as the allure of television and the internet, you require at least seven and a half hours of sleep per night.


After you fall asleep, you should go through four stages of increasingly profound, dreamless sleep before entering a fifth stage of dreaming, which is characterized by rapid eye movement. Throughout the night, you will alternate between non-REM and REM sleep. The arrangement of these cycles, known poetically as “sleep architecture,” tells whether you are getting the restorative sleep you require to feel and be well. Stage 1, the lightest sleep, should account for no more than 5% of total sleep time and should occur only at the start of the night. Stage 2 sleep, which is still quite light, should account for 55 to 60% of the TST. Deep sleep, often known as “slow wave” sleep, occurs in Stages 3 and 4 and accounts for around 20% of the TST. As Shakespeare put it, this is the kind of sleep that “knits up the raveled sleeve of concern.” You will not feel rested the next day if it is shortened or missing. Your sleep study records the duration spent in each stage, both in minutes and as a percentage, as well as the latency between the time you fall asleep and the time you reach Stage 4 and REM sleep.


Arousals, which are sleep disturbances lasting 3 to 15 seconds, can occur spontaneously or as a result of sleep-disordered breathing or other sleep disorders. Each arousal causes you to return to a lighter state of sleep. If the arousal lasts more than 15 seconds, it is classified as an awakening. Arousals are normally not noticeable to you, but awakenings may be. The study records the number of arousals and awakenings and reports them as a total number and as a frequency per hour of sleep, which is referred to as an index. The higher your arousal index, the more fatigued you are likely to feel, albeit people’s tolerance for sleep disturbances varies. Some people can become chronically sleepy with as few as five arousals per hour. In the most severe cases of SDB, the index might reach 100 or more.


The arousals and awakenings that occur when anything goes wrong with your breathing while you’re sleeping can be caused by a malfunction in the central nervous system–that is, the brain “forgets” to breathe–but are more usually caused by a mechanical cause. (There are also “mixed” episodes that involve both components, but these are uncommon.) When the soft structures in the back of the throat collapse into the airway, it reduces the amount of air that enters your lungs and, as a result, the amount of oxygen in your bloodstream. In an apnea, airflow is reduced by at least 80%; in a hypopnea, the reduction ranges from 50% to 80%. The report shows the number of times each occurs as a total for the night and as an index of events per hour. This latter figure is known as the apnea-hypopnea index or the respiratory disturbance index. A score of 5 to 14 suggests a low amount of breathing–and sleep–disruption. A score of 15 to 30 is considered moderate; a score of 30 or above is considered severe. Desaturations, which are caused by reductions in blood oxygen levels, are also measured and classified. Normal saturation is around 95%. Desaturation to 86 percent is considered mild, 80 to 85 percent is considered moderate, and 79 percent or less is considered severe.


Goldberg emphasizes that these indices are not hazy notions, but rather actual data generated by easily measured electrical circuits. Nonetheless, they do not reveal the entire tale. A machine cannot determine the severity of your sleep apnea (or the type of treatment you require). Your doctor will look at the figures, but they must be placed in a clinical context. As a result, it’s critical that you keep track of and report on your daily experiences. Do you fall asleep when reading or watching TV? This is a relatively minor level of impairment and may not be cause for alarm. Or are you trying to sleep–or falling asleep–while doing things like eating, chatting, walking, and driving? That qualifies as a medical emergency. So don’t get too caught up in the numbers; they’re extremely useful, but they don’t replace speech.


In addition to the factors mentioned above, the polysomnogram examines your heart rhythm to see if there are any irregularities. The examination of limb movement is also an important element of the study, as leg movement can be a sign of another sleep problem. And, yes, someone is listening to–though not measuring in decibels–your snoring, which is likely what drew you to the lab in the first place.