Sleep Efficiency and Latency
The ratio between the total sleep time and the total recording time, or TRT, is called the sleep efficiency. People who have significant difficulties in either initiating or maintaining sleep have diminished sleep efficiency, which can be related to various conditions and disorders, including depression. The number of minutes between the time the light is turned out and the person falls asleep is the sleep onset latency. Normally, it takes about 15 minutes to fall asleep. A significantly shorter onset latency–the proverbial falling asleep as soon as your head hits the pillow–might seem desirable, but in fact is an indicator of sleep deprivation. This could suggest a disorder, or it could be just a sign to slow down. Regardless of the demands of work and family, and the enticements of television and the web, you need at least seven and a half hours of sleep a night.
Once you drop off, you should progress through four stages of increasingly deep, dreamless sleep and into a fifth stage during which dreaming–characterized by rapid eye movement–occurs. Over the course of the night, you will cycle repeatedly between Non-REM and REM sleep. The structure of these cycles–poetically known as “sleep architecture”–reveals whether you are getting the restorative sleep you need to feel and be well. Stage 1, the very lightest sleep, should only be 5 percent of the total sleep time, and should only occur at the beginning of the night. The still fairly light Stage 2 sleep should represent about 55 to 60 percent of the TST. Deep sleep–also known as “slow wave” sleep–takes place in Stages 3 and 4, and should account for about 20 percent of the TST. This is the form of sleep that, as Shakespeare wrote, “knits up the raveled sleave of care.” If it is truncated or absent, you will not feel rested the next day. Your sleep study measures the time spent in each stage, in minutes and as a percentage, and the latencies between the time you fall asleep and the time you enter Stage 4 and REM sleep, respectively.
Arousals and Awakenings
Arousals–interruptions of sleep lasting 3 to 15 seconds–can occur spontaneously or as a result of sleep-disordered breathing or other sleep disorders. Each arousal sends you back to a lighter stage of sleep. If the arousal last more than 15 seconds, it becomes an awakening. You are usually not aware of arousals, but may be aware of awakenings. The number of arousals and awakenings is registered in the study, and reported as a total number and as a frequency per hour of sleep, which is referred to as an index. The higher the arousal index, the more tired you are likely to feel, though people vary in their tolerance of sleep disruptions. As few as five arousals per hour can make some people feel chronically sleepy. In the worst cases of SDB, the index can be 100 or more.
The arousals and awakenings that occur when things go awry in your breathing while you're asleep can arise from a glitch in the central nervous system–i.e., the brain “forgets” to breathe–but more commonly have a mechanical cause. (There are also “mixed” episodes, involving both elements, but these are relatively rare.) The mechanical malfunction occurs when the soft structures in the back of the throat collapse into the airway, reducing the amount of air that makes its way into your lungs and, as a consequence, the amount of oxygen in your bloodstream. In an apnea, airflow is reduced by at least 80 percent; in a hypopnea, the range of the reduction is 50 to 80 percent. The report indicates the number of times each occurs, as a nighttime total and as an index of events per hour. This latter number is referred to as either the apnea-hypopnea index or the respiratory disturbance index. An index of 5 to 14 indicates a mild level of breathing–and sleep–disturbance. From 15 to 30 is moderate; greater than 30 is severe. The associated drops in blood oxygen levels, known as desaturations, are also measured and categorized. Normal saturation is around 95 percent. A desaturation to 86 percent is mild, a reduction to 80 to 85 percent is moderate, and a drop to 79 percent or less is severe.
These indexes, Goldberg stresses, are not fuzzy concepts, but hard data generated by easily measured electrical systems. Nevertheless, they don't tell the whole story. The severity of your sleep apnea (and what treatment you need) can't really be gauged by a machine. Your doctor will look at the numbers, but will need to put them into a clinical context. Consequently, it's vital that you track and report what you experience from day to day. Do you get sleepy while reading or watching television? That's a very mild level of impairment, and might not be cause for concern. Or are you fighting off sleep–or falling asleep–during activities such as eating, talking, walking, and driving? That's a medical emergency. So don't get too hung up in the numbers; they're enormously helpful, but they don't replace words.
In addition to the items covered above, the polysomnogram looks at your heart rhythm, and determines if there are any abnormalities. Another important part of the study is the assessment of limb movement, since leg movement can constitute another sleep disorder. And yes, someone is listening to–though not measuring in decibels–your snoring, which is probably the symptom that got you to the lab in the first place.