After the test is completed a "scorer" analyzes the data by reviewing the study in 30-second "epochs". The score consists of the following information: Onset of sleep from time the lights were turned off: this is called "sleep onset 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. This may be because of sleep state misperception, drug effects on brain waves, or individual differences in brain waves.) 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 7 channels: EEG (usually 4 channels), EOG (2), and chin EMG (1). From this information, each 30-second epoch is scored as "awake" or one of 4 sleep stages: 1, 2, 3, and REM, or Rapid Eye Movement, sleep. Stages 1–3 are together called non-REM sleep. Non-REM sleep is distinguished from REM sleep, which is altogether different. Within non-REM sleep, stage 3 is called "slow wave" sleep because of the relatively wide brain waves compared to other stages; another name for stage 3 is "deep sleep". By contrast, stages 1 and 2 are "light sleep". The figures show stage 3 sleep and REM sleep; each figure is a 30-second epoch from an overnight PSG. (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 medication), alcohol taken before bedtime, and sleep deprivation.) Any breathing irregularities, mainly apneas and hypopneas. Apnea is a complete or near complete cessation of airflow for at least 10 seconds followed by an arousal and/or 3% (although Medicare still requires 4%) oxygen desaturation; hypopnea is a 30% or greater decrease in airflow for at least 10 seconds followed by an arousal and/or 4% oxygen desaturation. (The national insurance program Medicare in the US requires a 4% desaturation in order to include the event in the report.) "Arousals" are sudden shifts in brain wave activity. They may be caused by