Measuring Sleep Quality

For all tests performed on Practical Sleep Insights, sleep quality is measured based on time in each sleep stage. The sleep stages are first recorded using an Apple Watch, which is among the best and most accurate instruments for measuring sleep without a comprehensive sleep study.

The Apple Watch measures sleep by tracking various sleep metrics through its sensors and algorithms. While it doesn’t directly measure specific sleep stages like a professional polysomnogram, it estimates sleep duration and quality based on accelerometer motion and heart rate data along with machine learning algorithms. The sleep stages are broken down into:

1. Awake: The watch identifies periods when you’re awake, detecting movements and changes in heart rate indicative of wakefulness.
2. Core Sleep: Characterized by reduced movement and a slightly lower heart rate, this stage typically represents lighter sleep.
3. Deep Sleep: Marked by slower heart rate and minimal movement, deep sleep is considered crucial for physical recovery.
4. REM (Rapid Eye Movement) Sleep: This is the stage associated with dreaming. During REM sleep, heart rate increases, and the watch may detect more erratic movements. REM is considered crucial for mental recovery.

The sleep score on this site is calculated based on

Sleep Score = REM + Deep Sleep - Time Awake (All in minutes)

I’ve found that this sleep score stays relative consistent with low variance when I don’t change my daily habits and sleep parameters. The sleep score is also a good reflection of how rejuvenated I feel the next day.

REM contributes to memory consolidation and mental recovery, so more time in REM results in a higher sleep score. Deep sleep provides physical recovery, and time in deep sleep also increases the sleep score. On the other hand, waking up in the middle of the night detracts from sleep quality and will decrease the sleep score.