Garmin Metrics Part 3: Sleep Tracking

Did you know that you spend about 26 years of your lifetime, or 1352 weeks (that is, 229,574 hours!) asleep? If the average life-expectancy in the US is 78.7 years1 that means you spend about 1/3 of your life asleep. Sleep is one of the most important factor contributing for our overall health and well-being, and greatly influences many bodily functions and homeostasis. Chronic lack of sleep is associated with an increased risk of cardiovascular disease, obesity and mood disorders2. People have become more and more interested in learning about the effects sleep on their health and performance, and wearable devices companies have tried their best to answer this call by developing methods for estimating time spent in different sleep stages. There are devices available that are specifically manufactured for the purpose of gathering data on sleep and recovery, but almost all new fitness trackers come with some kind of means for sleep tracking, and Garmin is no different in that!

Measuring vs. Estimating

Keep in mind that once again, like any other metric you can get from your wearable device, the sleep metrics are also estimates, and thus can only give you an idea of your actual sleeping patterns and behaviors. Polysomnography is the gold-standard for measuring sleep typically used in laboratory or hospital settings. With polysomnography, sleep stages and cycles are measured from the recordings of expiration, blood oxygen saturation (%SpO2), body position, ECG brain waves, breathing rate and effort; muscle EMG, eye movement, heart rate (HR)3; and often heart rate variability (HRV). Advanced Sleep Monitoring (ASM) is a feature found in many of the newer Garmin devices that have an optical heart rate sensor, i.e. are able to record HR from your wrist4. Unlike polysomnography, Garmin devices estimate sleep stages from your wrist movement (if you are wearing your watch in bed) using accelerometry; and the wrist-based HR and estimated HRV, which is derived from your HR using a patented technique by Firstbeat. Some of the more advanced Garmin devices also provide estimates of your %SpO2 using the same optical sensor.

Requirements for Obtaining Garmin Sleep Data

The first step is to check whether your device has the ASM feature (essentially, is it able to record HR from your wrist). You can find more information on the compatible devices on Garmin’s website. According to the manufacturer’s instructions, the Garmin Connect mobile phone app must be up-to-date, and the user information contain a valid birth date and the user’s typical sleep schedule4. The device must be worn at least 2-hours before bedtime, with the HR sensor enabled4. Make sure the device is snug on your wrist but not too tight to be uncomfortable. If you have more than one Garmin device paired with the same app, make sure that you are wearing the one set as the “preferred device” in the app. To obtain more accurate sleep data, wear your watch in bed consistently.

What Kind of Information Can You Get?

  • Sleep Stages
    • Light Sleep – the stage where you enter first after falling asleep; HR and breathing are slower than when awake. You are still sensitive to the external stimuli and may wake up easily.
    • Deep Sleep – the stage with the least movement; HR and breathing slow down. This sleep stage is important for recovery, restoration, and repair of the tissues.
    • REM Sleep – the stage where you dream; brain activity is almost at the level of when you are awake. This sleep stage is important for forming memories and processing information.
  • Movement Timeline – shows the different sleep stages and how much you moved around in the middle of the night
  • Seven-day Sleep Patterns – shows the average for the different sleep stages, time spent awake and the times you went to bed and got up throughout the week.
  • Bonus (in some devices)
    • Pulse Oximetry – estimated blood oxygen saturation (%SpO2) with a combination of red and infrared lights on the back of the device. Typical values are 95-99% at rest.
    • Respiration– based on the estimation of HRV, as changes in breathing pattern influence HRV.

Should You Trust This Information?

It’s cool how much interesting data you can get from today’s wearable devices! The data can be useful and provide feedback on your health-related behaviors and help in recognizing patterns and guiding lifestyle changes. However, it is always good to keep in mind that these measures are not telling the absolute truth as they are only estimates and thus, contain some degree of error. The Garmin ASM feature is not designed to be a scientific or medical instrument or a substitute to polysomnography. At the end, it is pretty much up to the consumer how much emphasis they put on the information obtained from wearable devices. If you think the data is useful, go ahead and use it, but do it consistently.

References

  1. CDC [Internet]. United States [cited April 23, 2021]. Life expectancy. Available from https://www.cdc.gov/nchs/fastats/life-expectancy.htm
  2. CDC [Internet]. United States [cited April 23, 2021]. Sleep and chronic disease. Available from https://www.cdc.gov/sleep/about_sleep/chronic_disease.html
  3. MedlinePlus [Internet]. United States [cited April 23, 2021]. Polysomnography. Available from https://medlineplus.gov/ency/article/003932.htm
  4. Garmin [Internet]. United States [cited April 23, 2021]. What is advanced sleep monitoring in Garmin Connect? Available from https://support.garmin.com/en-US/?faq=mBRMf4ks7XAQ03qtsbI8J6
Minttu Hukka, MS
We help endurance athletes (from beginners to pros) train and cross the finish line faster and injury-free!

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