In order to calculate sleep quality, Pillow takes into account body motions during sleep, as registered by the device's accelerometer and gyroscope, and also the noise level as recorded by the device's microphone. These data are processed by a scientifically backed algorithm in order to calculate your sleep quality index. As you keep using Pillow, sleep quality is adapted to your personal sleep patterns and your readings will become more accurate.
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Overall quality on average could range from 25% for a truly bad, short and disturbed sleep, to about 90% for a great sleep. Values outside these bounds would represent extreme and somewhat rare conditions (extreme disturbances, chronic sleep disorders or a very unfriendly sleep environment).
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In terms of sleep stage durations, according to research, the generally accepted optimal values for each sleep stage are 5% awake, 25% REM, 35% deep, and 35% percent light sleep for an adult, although these may vary from person to person depending on sex, age, weight, health condition, sleep environment etc.
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