Morning or Night for heart rate variability (HRV) measurement?

Technology for Heart Rate Variability (HRV) measurement at rest is getting better every day. From our own approach using the phone camera in HRV4Training (validated, and independently validated), to the Oura ring, Polar Vantage V2 and Scosche Rhythm24 (plus of course, good chest straps like Polar H10), it has become really easy to capture high-quality HRV data.

In a recent blog, I discussed the importance of using an accurate tool, sampling at the right time, and interpreting your data with respect to your normal values.

I have often argued that provided you know what you are doing, morning and night measurements are equivalent in the long run.  This does not mean that there are no differences on a daily basis (we'll get to that), but it means that these are both reliable way to assess baseline physiological stress.

Check out an example of a few months of my own data, with annotations, here

Application of interest

We first need to define our application of interest.

Both heart rate and HRV can be used for different applications and measured under different conditions. In this blog, our application of interest is determining chronic physiological stress level, which derives from combined strong acute stressors (e.g. a hard workout, intercontinental travel) and long-lasting chronic stressors (e.g. work-related worries, etc.).
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By measuring the impact of various stressors (e.g. training or lifestyle) on our resting physiology (HR and HRV), we can make meaningful adjustments that can lead to better health and performance.

Latest research

Back to morning vs night. Apart from my anecdotal data above showing a very good match between the two, what does the research say?

​In the latest paper looking at this exact question, "Evaluation of nocturnal vs. morning measures of heart rate indices in young athletes", Christina Mishica and co-authors report that "heart rate and RMSSD obtained during nocturnal sleep and in the morning did not differ".

We can even look at the data for both heart rate and rMSSD, a marker of parasympathetic activity (the same feature used in HRV4Training or in the Oura ring), which indeed show great agreement:

You can find the full text of the paper here.

Important differences

As I have tried to cover in this and other blog posts, morning and night measurements can be used to capture baseline physiological stress in response to acute and chronic stressors. Both methods have been used in different studies resulting in the same outcomes in terms of the relationship between HRV, training load, and recovery (see an overview here).
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While long term trends will be similar between these two methods, there are a few differences to keep in mind, mainly linked to these aspects:

• Workout (or other stressors) timing: if you exercise in the evening (or experience a late stressor, such as a large late dinner or alcohol intake), your HRV will take some time before going back to normal, and therefore it might be lower during the night, even if in the morning it’s all back to normal. This means that if you work out at different times of the day (sometimes in the morning and sometimes in the evening) a morning measurement might be better suited for you. On the other hand, if your training schedule is fairly similar across days, then night measurements will not reflect any differences and will work as well as morning measurements. Long-term trends will be similar between methods, but the acute or day-to-day response might differ based on stressor timing. Associated to this, it is my opinion  that it might make more sense to measure your physiological stress after the restorative effect of sleep, and not during, if we want to use the data for daily guidance (actionability) more than retrospectively. 
• Arrhythmias: in the context of measuring HRV, arrhythmias are artifacts. As I have described elsewhere, a single beat out of place will cause a disruption and artificially increase HRV. Normally, when we have such isolated events, we can deal with them and provide accurate estimates of HRV. However, if the issues are more frequent, and happen every few seconds, there is nothing to do and simply HRV cannot be correctly determined. Unfortunately, if your arrhythmia is frequent during the night, there is no point using a device that measures as you sleep. In this case, the only way to measure HRV is to take a morning measurement during a period in which you have no or fewer ectopic beats. This is not to say that devices using night measurements are inferior in terms of artifact detection or removal. However, in the morning you have control, you can wait a bit, you can assess if the measurement was artifacted, etc. — in the night your data will be impacted by ectopic beats and there is really nothing we can do. These issues should be carefully evaluated in sports settings as athletes tend to have a higher prevalence of ectopic beats, especially in the context of endurance sports.
• Parasympathetic saturation: parasympathetic saturation refers to a situation in which parasympathetic activity is particularly high, but this is not reflected accurately in HRV data. Parasympathetic saturation is a rare event that can happen in elite endurance athletes during high load training blocks, and you can learn more here. Depending on how you measure your HRV, you could be proactive and collect data that is less likely to be affected by the issue of parasympathetic saturation. In particular: if you measure your HRV during the night, there isn’t anything you can do. Hence you should use the procedure explained in the blog post here to determine if parasympathetic saturation is likely in your own case. In this case, it might be preferable to use a morning measurement. If you measure in the morning, you can measure while sitting (or standing), so that you add a little stress on your body and potentially prevent the issue of parasympathetic saturation, as recommended by Andrew Flatt

Practical takeaways

What are the takeaways here?

In my opinion, when it comes to the data, there is no advantage in using one method or the other. However, if you prefer to wear something over the night, get a device that does so. If you prefer not to wear something during the night and just to take a measurement in the morning, then go that way.

If your athlete can’t be bothered to take a morning measurement, get a device that tracks HRV during the night. If you are not sure this is for you, you can use your phone camera and invest as little as 10$ in measuring your physiology daily with an independently validated HRV app such as HRV4Training.
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Once again, if you like wearables, they can clearly capture high-quality HRV data as we sleep. Just make sure to use one that gives you the full night of data, as otherwise HRV measurements won't be reliable (as I discuss in greater detail here). My recommendation would be the Oura ring for this exact reason: most other sensors will provide automatically collected sporadic data points (e.g. 5 minutes during the night, like the Apple Watch does), which unfortunately are noisy and do not reflect underlying physiological stress very well.

As an alternative, you can get the same data with a morning measurement taken with HRV4Training, the only validated camera-based app. Pretty simple and cost effective, as long as measuring in the morning works in your daily routine.

This is great news as we all have different reasons to use one method or the other (cost, preference for passive measurements, etc.), but as long as we use valid tools, the same physiological processes can be captured over time.

This consistency will help us move forward in many areas of research in which these sensors or apps are currently deployed.

Stay in touch.