How Sleep Tracking Wearables Can Help Shift Workers Uncover Hidden Fatigue

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Note: this article focuses on the topic of general wellness tracking, and not on medical diagnosis. When it comes to concerns about chronic fatigue, suspected clinical disorders and individual medical cases, please consult with a professional healthcare provider.

Introduction: Subjective vs objective rest data

Summarizing in advance: Most of us, when we feel "well-rested," are in fact using a subjective assessment, and a sleep tracking wearable (or a cheaper alternative) is often the best value purchase we can make regarding sleep. It has nothing to do with self-delusion on a philosophical level; the perception of our own degree of rest is demonstrably less accurate the more sleep-deprived we are, which is a uniquely counter-intuitive twist.

To put it in perspective: when you wake up from a 6 hour day-nap, grab a coffee and rate your sleep as "ok, could have been better", there's a high chance that either your Oura Ring or Whoop strap is registering a readiness score of 42%, near-zero% of deep sleep and a heart rate variability (HRV) far below your own personal average. Only one of these readings can be correct, and it isn't the one that happened inside your skull.

It's important to qualify that statement, because the counter-argument is reasonable: sleep tracking wearables aren't medical devices, and the proprietary algorithms used to determine your sleep quality are based on motion tracking and heart rate to a far greater extent than standard polysomnography used in clinical settings. Take the information for what it's worth, and consider it as a useful trend rather than an absolute measurement for any given night.

What to track: REM, deep sleep and HRV

Not everything you see on a sleep tracking wearable is equally important. For a shift worker seeking to determine if their day-time naps are as restorative as they need to be, there are three broad indicators that can demonstrate this on a nightly basis.

• Deep sleep, also called slow-wave sleep, is the period of sleep during which the body carries out most of its tissue regeneration, as well as the clearance of misfolded proteins from the brain. During this phase, the body produces large amounts of human growth hormone, which plays a role in cellular repair and general metabolic maintenance. Adults should aim to get approximately 13-23% of total sleep time in deep sleep, according to clinical studies published in National Sleep Foundation journals, and shift workers are shown to regularly have trouble reaching this threshold. It is, for a shift worker, the most informative number on a sleep tracking wearable.

• Next, REM sleep: this phase is strongly associated with emotional processing, memory consolidation and increased oxygen consumption, and it comprises approximately 20-25% of total sleep time in adults. Due to the nature of fragmented day-time sleeping, shift workers frequently fail to get enough REM sleep to meet this threshold. This is due to the REM periods being shorter in daytime naps, especially early in the sleep cycle, and overall insufficient time spent asleep on a nightly basis.

• Finally, HRV, or heart rate variability, which is a measure of the variance between heartbeats. It is a strong indicator of your autonomic nervous system state: a high HRV when awake and a low HRV when asleep is generally indicative of good physical health. However, note that HRV is a highly personal metric, and that there is no universal standard to which you should compare your night-to-night or week-to-week measurements. This isn't an indicator to be evaluated on an absolute value, but rather a fluctuating sign of your general stress levels to be used over a long period of time.

Recognizing the "fake sleep" pattern

If you've used a sleep tracking wearable and you've seen a night of sleep where the total time spent asleep was high, but the deep and REM percentages were unexpectedly low, there's a good chance you've encountered "fake sleep".

Fake sleep is what happens to you when you're already tired, but you still need to get up for work or some other reason and decide to take a short nap: the time spent asleep is long enough for you to feel somewhat rested, but the wearable shows that either none or very little of that time was spent in either deep or REM sleep.

• Fake sleep is a consequence of either external or internal disruption of the sleep cycle. External disruption includes sleep interruptions such as morning light getting through the curtains or a ringing cell phone during the day.

• The wearable sleep stage tracker won't register these as time spent awake unless you actually get up and move, but they will affect your ability to reach deeper stages of sleep.

• Internal disruption is a consequence of your body refusing to accept that it's time to sleep at a certain time of the day. Even in the best conditions of an artificially induced dark environment, most people will experience an increase in cortisol levels throughout the morning which negatively impacts slow-wave sleep.

• One thing to note is that the accuracy of these devices' readings for REM sleep is generally lower than for other categories, according to most manufacturers and independent studies. Use common sense when interpreting your data: when looking at a single night's sleep, you should take the percentages of light, deep and REM sleep as approximations, with REM being an approximation with less precision.

Practical recommendations based on findings

It's pointless to simply collect this data without applying it to make useful changes in your routine. Here are some practical recommendations based on the findings mentioned above.

1. If your deep sleep percentages across multiple nights are consistently below the recommended thresholds, consider adjusting the room temperature where you sleep.Most sources suggest a room temperature between 60 and 67 degrees Fahrenheit to help facilitate deep slow-wave sleep.
2. If your REM percentages are similarly low, consider adjusting the total time spent asleep, as a lack of time is the most frequent cause of reduced REM for the shift worker.
3. If your HRV is consistently low from week to week, a good way to evaluate this is to assess whether you've taken proper active rest.
4. If you've taken a full day off and not done any strenuous exercise but it's not reflected in your week-over-week HRV averages, it's best to seek a medical opinion. It is generally advised against trying to "increase" HRV, as this can be a harmful practice. Looking at data from the last 2 weeks, and comparing it to the trend from previous months, is a safer option recommended by most wearable companies. A useful summary of this information can be found in the "Trends" section of the Oura app; in Whoop, it can be found under the "Monthly Performance Assessment".
5. Look at the data in relation to your shifts. Open the app on your wearable, navigate to the Trends section and manually set a time range for a period of weeks (in the Oura app). Mark the weeks that correspond to your shift patterns to see if any patterns can be identified between consecutive nights of inadequate deep sleep or reduced HRV.
6. Don't obsess over a high number every night. I'm going to say it right here, right now: trying to hit a target on a nightly basis for any number provided by a sleep wearable is counterproductive and can lead to reduced HRV and deep sleep percentages in the long run.

Use the information as a guide in the long run, rather than a target to hit on a nightly basis.

Conclusion & summary

A sleep tracking wearable isn't a cure-all or a substitute for professional medical evaluation if something is objectively wrong, but what it can give is an objective perspective on how well you're sleeping compared to how you feel about it. Deep sleep and REM percentages are strong indicators to be considered for a shift worker seeking to understand how much actual rest they get on average; use the HRV number to get a sense of how much stress you're under over an extended period of time. Take the information in context, use it to improve your lifestyle or working conditions if needed, and give yourself time to adjust. After a few weeks, most of these numbers should no longer be mysterious: they'll point you to the single obvious change you can make to improve your sleep quality every night, such as spending more time asleep or in a cooler room.