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.
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:
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
The interesting thing for me is how the relationship between age and recovery changes. It explains why we think as we get older it becomes harder to recover from training and we don’t quiet bounce back as we use to.
It also helps me understand the dynamics or training with different age groups. Some of the younger athletes I train with seem to be able to put more intense sessions in back to back and now I understand why they don’t feel the effects, their ability to recover is better.
Hi Kate. Good points! A couple of those were realisations also for me as I read the paper – one of the reasons for deciding to summarise it. Glad you found it useful. Simon.
Knowing this is helpful! I used to be able to train 2-3 days in a row before I took a rest day and up to 6 times a week; and could probably do 7. I’m only 36 but it seems even training 2 days in a row is sometimes too much where my body needs 2 days of recovery. I don’t believe my workouts are very intense but I do try and lift heavy but take longer breaks than I used to. I’ve also noticed I have to be careful when I do HIIT training because the last time, I did not recover well and ended up sick. I’m now a little nervous do one again! Maybe working out every other day is what my body needs now?
Hi Anjoli,
Yes indeed – recovery, especially from intense & resistance training definitely takes longer as you get older. I’m nearly 60 and I can now only cope with 1 intensive session per week. But if you have a solid base from years of training when you were younger, it doesn’t seem to limit your potential, and knowing your body definitely makes you feel better most of the time!
Hi Peter,
Why HRV scale above 15 – 60 (As Ive seen with other diagrams on RMSSD) & other HRV scales like the ithlete measurement scale 50 – 100 ish.
Many thanks Shane.
Hi Shane,
Aha! Good question. The raw RMSSD scale is neither user, nor scientist friendly as it is what is called a skewed distribution. Most studies deal with this by taking the natural log (Ln), but that gives a user unfriendly approx 1-5 scale so when we made the first HRV app back in 2009, we multiplied the Ln by 20 which gives a scale with a number of nice properties – it makes sense to people, is suitable for statistical analysis, and 1 pt is approx what’s called the smallest worthwhile change (SWC) so you don’t need a decimal point.
More at https://hrvtraining.com/2013/07/04/rmssd-the-hrv-value-provided-by-ithlete-and-bioforce/ from renowned HRV researcher Dr Andrew Flatt
Hope this helps,
Simon.
Hello Simon!
Looking at the circadian variation of the 18-24 I question the legitimacy of an out of the bed measure. This is a time when HRV still plunges fast. Wouldn’t a measure at 1130 be more reliable?
Hi Yves-Marie
Good to hear from you! Whilst I see your point about the sensitivity to time of day, it is still the most convenient point in most peoples’ routines and does help to reduce the variance caused by breakfast, caffeine, morning commutes, emails etc. So it’s probably the best compromise!
Hi Simon, I am new to HRV and am so pleased to have found your website and articles, which will help me train for an endurance cycle event in Aug 21. Thank you so much.
My brother is a long haul airline Captain with British Airways. The majority of his flights are overnight and East/West. I have yet to talk to him about HRV but his measurements would be interesting, I am sure. Regular time zone changes, stress of landing, sleep and diet disruption etc. What would your advice be to him as the most appropriate time to measure his HRV?
I am sure your advice would apply equally to long haul passengers.
Many thanks for your thoughts, Nick
Hi Nick
Thanks for the feedback, and I hope indeed that the 120+ articles on the site help you train for your endurance event.
I’m sure your brother’s measurements would be interesting – and telling due to the stresses you’ve mentioned. It’s not easy to give a definitive answer as to what time to measure.
The most important principle is to take a reading at the same point in your circadian rhythm, as described here https://www.myithlete.com/important-take-ithlete-hrv-readings-time-day/
The body clock only adjusts to time zone changes at a rate of ~1 hr per day, and it may be that he deliberately tries to remain on a home time zone or tries to fast forward to the new zone if remaining there for several days, but the circadian principle remains.
If that is not convenient, then I would say just to measure straight after waking, as you would at home, and watch the HRV improve as adjustment & recovery progress.
Hope this is helpful,
Simon.
Although HRV is clearly diminished by ageing, I’m wondering if some cardiac changes might actually improve it. Apparently about 25% of the population have a genetic potential to grow collateral arteries (a home-grown bypass) and I’ve done just that. 7 years ago I was fitted with a stent, which cardiologists now admit was a mistake. I’m just on 75 y.o. and have been running constantly for 57 years. For decades that was 10 km most days in about 42 minutes. Now I run intervals at speed (well, pathetic but fast for 75) pausing for HR recovery, 1 km morning & evening, and 4.7 km midday. Overnight I wear a Polar Ignite which claims to take a 4 hour overnight average reading. My 28 day average HRV in their metric is 92. Today it was 101. The average has crept up from about 84 over 3 months. Nowhere does Polar reveal how they make this calculation. The number seems wildly high for a 75 year old. It is unfair to ask since you have a competing product, but am I a freak from Krypton?
Hello Thor! Certainly sounds like you are a lifelong athlete, and as such I would not expect to see your HRV rise to a significant extent at this stage. One way to tell would be if your morning resting HR is also falling, then the rise in HRV is plausible. Otherwise it is more likely to be daily variation, without changing the underlying baseline. With ithlete Pro, we analyse several metrics to try to identify what might be causing rises (or more commonly, falls) in HRV.
It’s interesting that you say some people have the potential to grow collateral coronary arteries. In a BBC TV program on improving the human body, Professor Alice Roberts cited this as a desirable upgrade to avoid the worst consequences of coronary artery blockages (ie heart attacks). Apparently dogs have them from birth!
I am new to tracking my HRV and normally take my reading upon waking at 5am. My readings have been consistently between 62-90 depending on how hard I’ve pushed myself strength training the day before. My concern is ghat I read in your article ghat hrv is at its highest at night during sleep. If I fall asleep with my tracker on, my readings are around 17-19 at 11:30pm. I am well asleep as I lay down at 10. Should I be concerned at such a significant difference?
Hi Joyce,
I’m assuming the 62-90 waking readings are using ithlete, in which case they are perfectly fine and reflect a good level of fitness (https://www.myithlete.com/blog-heart-rate-variability-long-life-whats-connection/). With your tracker (you don’t say the make & model) it is likely that they are using a different scale to that of ithlete (https://www.myithlete.com/ithlete/scientific-foundation/), so you really can’t compare them. I would recommend just doing the morning measurements as they really tell you all you need to know about how recovered you are following the beneficial effects of sleep, and before any major stresses of the day!
best wishes,
Simon.
hi there, i am 48 and my average HRV on whoop is 19 i exercise regularly and consider myself quite fit. is this a worryingly low number please?
Hi Elliot. We really can’t comment on numbers produced by whoop. I don’t know what number system they are using or if they are accurate, and therefore if you need to be concerned. I would contact their support.
best wishes,
I have a Fitbit ionic and it just started measuring this. I’m a 48 year old female that is fit and runs about 15-20 miles a week (about a 9:50 pace). I was surprised to see I was only at 33. It was taken while sleeping. Should I try a different device?
Hi Dee,
I’m afraid we just can’t comment on numbers produced by other systems – we don’t know what algorithm they are using or if they are accurate, and therefore if you need to be concerned. I would contact their support for help.
If you do wish to try ithlete though we have a free 28 day trial available and are compatible with most HRM chest straps. So you could try that to see how your ithlete HRV number compares.
Let me know if you have any questions.
Best wishes,
Laura
Do you have normal data on persons = to or > 80
Hi Ed,
There’s data for people between 80 and 90 in this study:
https://www.myithlete.com/blog-heart-rate-variability-long-life-whats-connection/
Would be interested to hear how you compare!
I have an app that tracks hrv periodically 24/7 through my Apple Watch (hrv tracker app) My hrv is consistently significantly lower at night than day time. What might be the cause of this anomaly?
Hi Dave,
We really shouldn’t comment on data from other systems as we don’t know how they are measuring HRV, and so on. In addition, as far as we are aware wrist based devices haven’t been validated for HRV measurement. I would suggest reaching out to their support for more information.
Best, Laura
I have fallen asleep with my fitbit on twice. I have recieved hrv readings of 13 and 11. I am on medication to treat an overactive thyroid, could the medication be contributing to the low reading? I have been told I do not have signs of heart issues.
Hi Kristen,
Thanks for getting in touch. Unfortunately, we cannot comment on HR/HRV data from another source, i.e. Fitbit. I would advise a read of this post below regarding the suitability of such measurements from a wrist-based device.
How accurate are consumer pulse sensors?
If you have/start tracking upon waking with ithlete we would be glad to take a look and see if we can spot anything helpful.
Many thanks,
Laura
mine seem to follow the same rythym but jump as high as 150 for me around 1 in the morning. i am 46 years old and a runner…is that too high?
That really is very high! Do you get a green/amber/red indicator with that?
Hi Simon,
Reading this is interesting for “the general population” and of course, as a coach. There are a few questions regarding changing HRV, resting heart rate and going into peri-menopauze and menopauze.
The research on how to change your training plan as a female athlete before and during peri-menopauze is slowly finding its way down the coaching ladder.
Has there been any research on HRV in that regard?
i.e. ; HRV gets lower during transition and why this happens? (just one of the questions). Thanks in advance,
Best, Barbara
Hi Barbara
We haven’t found any research on HRV in healthy female athletes who are going through menopause, but there is some research to show that continuing exercise training can help to offset HRV declines caused by the change (and general ageing): https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2387062/ In my experience of 10+ years of daily HRV use, it has always been a valuable barometer of overall health – both day to day and in trends. The tricky part is figuring out what we need to change as we get older