Fitbit for tracking sleep: An honest review

Not all gadgets are bad for the sleep; some are ‘disruptive’ (technologically speaking!). We are talking about sleep trackers. These devices have penetrated our daily lives and have enabled us to account our sleeping practices. Not only that, if you suffer from insomnia, gathering data from these devices would give you and your doctor a head start.

In this blog, we will look at the efficiency of Fitbit as a sleep tracker. We (Refine Naturals) are not affiliated with Fitbit, and we have no conflict of interest to disclose.

What is sleep tracking?

Your sleep can be quantified in several ways. Have a look at Medical sleeptionary^[1]:

1. Total sleep time (TST): the actual sleep time while you were in bed
2. Time in Bed (TIB): time you switched off the lights till the time you woke up
3. Sleep efficiency (SE): The proportion of sleep out of the time you spent in bed (i.e., the ratio of TST to TIB)
4. Sleep onset latency (SOL): The duration of time from “lights out,” or bedtime, to the onset of sleep
5. Sleep stages: Your body goes through cycles of 4 phases: awake, REM (rapid eye movement), light sleep and deep sleep

How can smartwatches monitor sleep?

Fitbit models use accelerometer (which tracks motion patterns), oxygen saturation sensor and heart rate tracker for monitoring sleep.^[2] This is done by the flashy green and red light sensors you see on the back! When you haven’t moved for about an hour, your tracker or watch assumes that you’re asleep (based on the algorithm fed into it). While you’re sleeping, your device tracks the beat-to-beat changes in your heart rate, known as heart rate variability (HRV), which fluctuate as you transition between light sleep, deep sleep, and REM sleep stages.^[3] The same core hardware/software algorithm technology has been incorporated in all sleep-staging Fitbit models introduced since 2017.^[4]

1. Total Sleep time: How reliable is it?

When it comes to measuring total hours you slept at night, it is a cake walk for these devices. This was verified in a study done in the University of Tokyo, which checked how well does Fitbit Versa™ match up with a single channel portable EEG (electroencephalography).^[5] The device was an astounding 92% sensitive in detecting sleep.

2. Reliability of measuring Sleep stages: Far from Ideal

Fitbits give a breakdown of how much time you spent in each of the 4 stages. REM and Deep sleep are necessary for feeling rested and staying healthy. Surprisingly, we also spend some time awake in the middle of our sleep! Fitbit’s algorithms capture that.

While Fitbits have been shown to accurately measure the wake times^[4,6], they are not so perfect when it comes to detecting other sleep stages. Independent studies suggested that ‘Light sleep’ times are underestimated, while ‘Deep sleep’ and ‘REM sleep’ times are overestimated.^[4–6] It seems that measuring brain electrical signals (via EEG) is the only accurate way to assess sleep stages.^[6] Read more on the ‘Architecture of normal sleep’ here.

3. How useful is your Sleep Score?

Introduced in 2019, Sleep score intends to give users an easy metric to encapsulate their sleep quality. To get a higher score, you must have a longer total sleep and larger proportions of REM sleep and Deep sleep.^[7] Other minor determinants include resting heart rate, tossing and turning while asleep (together termed ‘Restoration’) and oxygen saturation variations.^[7] While it’s a fun feature, Sleep Score is not USFDA approved (yet..) when it comes to diagnosing sleep disorders.^[8]

4. Can I measure my oxygen saturation using Fitbit?

Simple answer: No. Fitbit only measures variation in your arterial oxygen levels (only when you are asleep), does not give a discrete reading. So, it can tell if your oxygen levels varied a lot while you were asleep. Though, this can suggest that you are having sleep apnea, but further testing needs to be done to diagnose it and Fitbit is not approved by regulatory authorities when it comes to diagnosing sleep apnea.

Newer Fitbit models even come with a smart alarm that does not wake you up when you are in the blissful stage of deep sleep. Instead, it starts waking you up 30 minutes before in subtle ways (how mommy woke you up as a kid!). But before you head to the market, read our blog on ‘Lord of the Bands: A Comparison of Sleep Trackers’

Overall, these devices are pretty reliable when it comes to tracking your sleep. Do you need all that data? Well, it surely convinces you to adopt healthier sleep habits. Read more on them here. Lastly, these devices are not a substitute for standard clinical polysomnography to properly diagnose sleep complaints and disorders.

At Refine Naturals™, we believe in a holistic approach for a better sleep. At Refine Naturals™, we believe “You Deserve Better than FINE!” 

References:

1. Sleeptionary - Definitions of Common Sleep Terms [Internet]. Sleep Found.2014 [cited 2020 Nov 23];Available from: https://www.sleepfoundation.org/sleeptionary-definitions-common-sleep-terms
2. Fitbit User Manuals [Internet]. [cited 2020 Nov 23];Available from: https://help.fitbit.com/articles/en_US/manuals.htm
3. What should I know about Fitbit sleep stages? [Internet]. [cited 2020 Nov 23];Available from: https://help.fitbit.com/articles/en_US/Help_article/2163.htm
4. Haghayegh S, Khoshnevis S, Smolensky MH, Diller KR, Castriotta RJ. Performance assessment of new-generation Fitbit technology in deriving sleep parameters and stages. Chronobiol Int 2020;37(1):47–59.
5. Svensson T, Chung U-I, Tokuno S, Nakamura M, Svensson AK. A validation study of a consumer wearable sleep tracker compared to a portable EEG system in naturalistic conditions. J Psychosom Res 2019;126:109822.
6. Stone JD, Rentz LE, Forsey J, Ramadan J, Markwald RR, Finomore VS, et al. Evaluations of Commercial Sleep Technologies for Objective Monitoring During Routine Sleeping Conditions. Nat Sci Sleep 2020;12:821–42.
7. What’s sleep score in the Fitbit app? [Internet]. [cited 2020 Nov 26];Available from: https://help.fitbit.com/articles/en_US/Help_article/2439.htm
8. Roy S. Clinically Validated Sleep Wearables Are Coming [Editor’s Message] | Sleep Review [Internet]. [cited 2020 Nov 26];Available from: https://www.sleepreviewmag.com/sleep-diagnostics/consumer-sleep-tracking/wearable-sleep-trackers/clinically-validated-sleep-wearables/