Since ithlete started, almost ten years ago, we have recommended you take your Heart Rate Variability (HRV) reading at the same time of day – just after waking up and before drinking tea / coffee or checking your emails and social media.

So why does it matter what time of day you take an HRV reading, and why can’t you compare a reading taken in the morning with one taken after lunch or in the evening?

Who, what & why?

Many biomarkers, such as heart rate and blood pressure vary throughout the day. Lack of comparability between measures taken at different times of day presents challenges in identifying trends signalling that a medical condition is getting better, or worse – for instance in the diagnosis of hypertension (high blood pressure). It also represents a challenge for the more widespread use of HRV, which otherwise has great potential to be used as a general physical and mental wellness index.

HRV is created by signals to the heart from the nervous system, and these signals show a 24-hr day-night variation in order to optimise functioning at the ideal time of day and to conserve energy and other resources for when they are most needed. All the body’s circadian timings come from the central pacemaker in the brain. Genetic variations determine whether you are a morning or evening person and how your mood is likely to vary.

Researchers from the Ulm Medical Center in Germany and the Perform Center in Montreal, Canada decided to investigate the 24 hr variation in HRV in healthy, depressed and physically sick patients to see whether they could find an underlying pattern that would allow measures taken at different times of day to be compared.

They did this by recording every single heartbeat over a 24 hr period and dividing the total recording into 5 minute intervals to produce 288 HRV measurements for each subject. Their preferred HRV measure was RMSSD, the same one used by ithlete, though they used the raw number, rather than the log scale used by ithlete.

What did they find?

The image below shows a typical 24 hr variation in 5 min values of HRV (RMSSD) for an individual in the study. The green line is the average 24 hr value, the grey line is a moving average, and the orange curve is what’s called a cosine curve. This curve often fits the daily variation in bio markers, and the relatively close fit to the moving average shows that it’s applicable to HRV too.

How does your HRV vary throughout the day

The next chart shows the daily variation pattern for nearly 1000 healthy people from different age groups from 18-24 to 55-67 years old:

How does your HRV vary throughout the day 2

What does it mean?

Some of the key observations made about the variation in HRV over the 24-hr period were:

  • HRV is highest at night (between about 10pm and 2am). This shows that the body is at its most relaxed, and in fact much of the important recovery and repair takes place during this period.
  • HRV is at its lowest between about 9am and midday. For many people this is the most active time of day, so it’s not surprising that the body’s rest and digest is mostly switched off during this period.
  • Both the mean (average) value of HRV and the night-day variation reduce as we get older. This parallels our reduced ability to recover as we age. The big reduction in night time HRV is noticeable in the 35-44 age group, and this is the age at which athletic recovery starts to take longer.
  • Lower average daily levels of HRV (RMSSD) in both people who were depressed, but also by people with low grade inflammation. This second finding is a very important one for athletes, because intense training triggers an inflammatory reflex, and this may be one of the main reasons why HRV is so good at detecting when you are not fully recovered.

Conclusions

This new study has shown day-night HRV variation to be substantial, with HRV almost twice as high around midnight than at mid-morning in younger people. Both the 24 hr average and the amount of variation decrease as we age, in parallel with our day to day recovery becoming less effective.

The curves show that changes in time of measurement can have a significant impact, for instance for someone aged between 35-44, if you take a reading at 10am compared to 6am, you can expect it to be 6 pts lower on the ithlete scale. That is substantial, and is why we strongly recommend you to take your readings all within an hour of your usual time. If you can’t manage to do that, e.g. after a very early rise or a weekend lie-in, it’s probably better to skip the measure for that day.

By Simon Wegerif

Circadian Rhythms of the Autonomic Nervous System: Scientific Implication and Practical Implementation 

 

 

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