Overnight HRV: Comparison of PPG to ECG

Problem:  Short-duration morning HRV measurements may be susceptible to exactly when you wake, your HR, breathing rate, thinking about work etc in that moment. Different apps report widely different values of HRV.  Perhaps more importantly, morning snapshot measurements do not record HRV over a physiologically meaningful interval.  The best case would be to record HRV 24-7, but this is not feasible with current wearable technology.  However, wearables can and do report night time HRV.  At least this is measuring vagal balance over a physiologically meaningful interval, namely, the sleep recovery timeframe.

Methods:  I recorded overnight HRV using Polar H7 chest strap connected to the iOS app HRV Logger, for 12 nights over more than a month.  HRV Logger stopped recording on 2 nights, perhaps due to loss of connection with the H7 or other errors, so I have 10 nights of complete data.  The Polar H7 chest strap directly measures electrical activity (ECG) and is considered the gold standard for at-home HRV measurements.  

I also wore a Fitbit Inspire 2 each night and downloaded the data file from Fitbit.com at the end of the month.  Fitbit uses a green light PPB (photoplethysmography) sensor to measure blood flow changes at the wrist and infer heart rate and HRV.  

HRV Logger was set to 5 minute recording windows and RR-interval correction was set at 25%.  Fitbit also reports HRV in 5 minute recording windows.  However, the 5-minute interval windows are only approximately aligned as they are based on offset starting times.  HRV is reported as rMMSD.  

Results:  At this point, I am reporting preliminary data from 3 representative nights.  

Night of 9/19

Summary

	Fitbit	Polar
mean	38.5	41.48
std dev	6.75	8.44

Night of 9/21

 

Summary

	Fitbit	Polar
mean	39.82	44.32
std dev	6.94	9.23

Night of 9/28

 

Summary

	FitbitInspire2	PolarH7
mean	35.06	39.57
Std dev	5.31	7.80

Conclusion

An examination of the time course data shows that Fitbit was able to pick up the broad changes in HRV during the night.  It appears Fitbit misses spikes in HRV that the Polar strap is able to detect.  Although it is also possible that the Polar strap is biased by movement artifacts, it seems most likely that either Fitbit is not sensitive enough to detect short-duration changes in HRV, or a Fitbit error-correcting algorithm is too aggressive at removing short changes in HRV.

In addition to being less sensitive to variations in HRV (as measured by standard deviations), Fitbit also consistently records a lower average nightly HRV than the Polar strap.

The overall R squared value is only 0.38, however this statistic probably underestimates the correlation of time series data due to the fact that the 5-minute windows are not perfectly synchronized.  Even so, R squared of 0.38 is a higher correlation that I was able to achieve using the Polar H7 with 2 other apps.   

Fitbit Data Analysis: HRV and Temperature

 Comparison

Between April/May 2020 and August/September 2021....

    .... my resting HR went from average 52.4 to 55.3

    .... my deep sleep went from average 75.6 to 91.5 minutes

Nightly HRV analysis

Night of Sep 4-5th:  Morning HRV was 62 in Elite HRV, 82 in ithlete, 41 in Fitbit, and 64 in HRV logger.  

Night of Sep 5-6th: Elite HRV was 66, ithlete was 82, Fitbit was 51, HRV logger was 54.  94% of time HR was below resting.  

Sep 6th-7th (but only 6th is shown, due to not downloading Sep 7th data yet...)

Comparison of cheststrap (30 sec sample data smoothed to 5 minute window, Fitbit HRV sampled on 5 minute intervals).  Fit looks pretty good.

Recently found out Fitbit records temperature as well.  There is a diurnal rhythm, with interesting variations.  Sep 4th and 7th were both early morning outdoor exercise (jogging and hiking, respectively).  

More temperature details:

The Inspire 2 temperature sensor is at least directionally-accurate!

The nightly data ("Computed Temperature" file) has what looks like temperature in Celsius. For me, it is 31-32 Celsius at night, which is around 90 F. This is the data that is reported as Skin Temperature in Health Metrics.

Interestingly, there is also continuous 24 hour temperature data at 1 minute intervals ("Device Temperature" file). This data appears to be recorded as variation (+/-) from some baseline, as the numbers are always between +3 and -8 for me. Again, probably Celsius. I plotted my data from the last few days, and the times when I was exercising outside in the sun were consistently +2 degrees above other times that I would guess were cooler.

I don't know if the temperature measures are absolutely correct, but they do at least make sense and could potentially be used to identify unexpected temperature anomalies.

If anyone else is trying to download their data, note that you have to request it on the Fitbit account setting website, and then confirm the email.

Sleep Apnea: Research and Self-Experimentation Results

 Abstract

I tested several sensors that claim to measure sleep apnea through optical measurement of blood oxygen content.

Background

Blood oxygen arterial saturation (Sa02) is approximated by Sp02.  At 5,000 feet elevation, Sp02 should be above 92%. Values under 90% are considered low.  Hypoxemia is dangerously low blood oxygen, defined as Sp02 <88% for more than 5 minutes.   (Hypoxia is low tissue oxygen levels)

An oxygen saturation level over 95 percent is considered normal. Anything below 92 percent oxygen may be a sign of breathing problems during sleep, such as sleep apnea or another disorder like severe snoring, COPD or asthma.  (Low levels can also be caused by anemia.)

Sleep apnea is characterized by a cessation of airflow lasting 10 seconds or more. Hypopnea (shallow breathing resulting in desaturation) is a decrease in airflow lasting 10 seconds or more with a 30% oxygen reduction and a 3 to 4% desaturation from the baseline. It is not uncommonpatients with sleep apnea to desaturate below 88%. 

Note: low Sp02 is an indicator of sleep apnea and is not necessarily the mechanism of the bad effects.  It may be possible to not have hypoxemia, but still have interrupted sleep.  

Sleep apnea and hypopnea can cause small awakenings that disrupt normal sleep cycles.  This can cause irregular heart beats, high blood pressure, blood sugar excursions, strokes, heart attacks, etc.  Most common effect is to create chronic, elevated stress response.  

Results

- Fitbit Charge 2 never showed any apnea episodes

- Biostrap showed several, including some that my wife didn't remember

- Wellue 02 ring showed several mild episodes per night.  Based on other testing, this sensor seemed potentially the most accurate, but location on finger wasn't comfortable.  Also ended up not being accurate because I sometimes sleep on my arm, cutting off circulation, which reduces Sp02.

Conclusion

I have been diagnosed with mild sleep apnea and my wife has no known apnea.  We both tried the Sp02 wearable sensors and they consistently showed the same level of apneas between us, either none (Fitbit) or mild (Wellue 02 Ring).  Based on the lack of a clear and actionable signal from these sensors, I no longer use them to track apnea events.  Instead I focus on trying to minimize possible breathing issues and promote general good sleep hygiene to try to improve restfulness. 

What Do Carnivores Dream About?

Anyone with a house cat already knows the simple truth of a new study on the comparative physiology of sleep across mammals. Carnivores sleep much more than would be expected compared to herbivores and omnivores:

Carnivores:

Herbivores:

Omnivores:

Of course, these data don't answer the original question, but only raise more questions.

This website uses the data to conclude that humans should eat a vegetarian diet, because human sleep requirements match those on the Herbivore sleep regression, and we don't get as much sleep as other omnivores of the same body weight.  But based on discussion in The Story of the Human Body: Evolution, Health, and Disease by Daniel E. Lieberman humans in their modern form -- i.e. that would be recognizable as human today -- cooked their food, which consisted of vegetables and meat and fish.  None of the other animals in this study cook their food.  Perhaps our higher quality diet allows us to spend less time digesting and more time alert?

But herbivores spend very little time asleep!  In fact, from this data, one might conclude that a limit on the size of herbivores is the number of hours in a day.... the largest herbivores spend almost the entire day (and much of the night) awake and, probably, eating.  In contrast, carnivores sleep the most, presumably because they can satisfy their nutritional requirements with less time and effort.

Why don't humans sleep more?