In longevity circles, high heart rate variability (HRV) means top health. Well, I did a sleep apnea training last year, and I was surprised to learn that in that domain, the world's best experts interpret high heart rate variability at night as a possible sign of significant pathology.
The devil is in the details, though, and one could say that it's a matter of interpretation and physiological reading parameters. Nevertheless, I thought that I would clarify because I already encountered someone whose Oura ring was giving the highest heart rate variability ever recorded in what was one of her worst nights ever due to an asthma attack. We thought the ring was bogus, but maybe it was just faithfully reporting what it was reading...
First, I'll clarify what happens while we sleep.
There are 4 stages during a sleep cycle. In the first stage, there's still some vigilance, and this is when snoring people can hear themselves. Muscle tone decreases.
During the second stage, muscle tone goes down even further. However, there can be sympathetic activity with blood pressure variations during what are called "K complexes" that appear during an EEG (electroencephalogram) which come from activity in excitatory cortical neurons. Despite the fluctuations, the important thing to remember, is that there's parasympathetic dominance with a decreased sympathetic activity.
In the third stage, there's very little muscle tonicity. It's the typical deep sleep when it's extremely difficult to wake up. There's dominant parasympathetic activity with reduced sympathetic activity, decreased heart rate, decreased blood pressure, slower respiratory rate, there's a moderate drop in cardiac output. There's even further decreased muscle tone.
When we transition from stage 1 and 2 towards stage 3, there are parasympathetic outbursts and that can translate in certain bradycardias and atrioventricular blockages (when the electrical activity of the heart goes very slowly or is blocked altogether).
The last stage is REM (rapid eye movement) which has two activities: the tonic component and the phasic component. The tonic component is always in the background, and it refers to the fact that we are actually "atonic", that is, we have no muscle tonicity. There's complete atony of all the striated muscles, except for the diaphragm (we're still breathing). There's a very strong cholinergic vagal tonus, with decreased heart rate and blood pressure. There are parasympathetic bursts which can favor sinus pauses (pauses in the heart's pacemaker).
REM's phasic component refers to cholinergic activity in some centers of the brain stem, which activate the muscles of the eyes and brain centers related to visual imagery (dreams). There are sudden movements of the face and extremities. You can see twitches and micro awakenings. Here, we can have sympathetic bursts which favor these micro-awakenings. Parasympathetic activity is inhibited. People predisposed to arrhythmias can have ventricular extrasystoles.
During REM, there is increasing sympathetic activity and reduction of parasympathetic activity. Some call it autonomic ("neurovegetative") storms. There's variability and there's cardiorespiratory instability. There's intermittent low oxygen which can favor increased blood pressure, which can become permanent in people with obstructive sleep disorders such as sleep apnea.
Brain activity variations during sleep have an important role in heart function, specifically on heart rhythm, blood pressure and coronary blood flow. Brutal fluctuations during REM are well tolerated in healthy people, but those with heart disease are at risk of arrhythmias, ischemia and heart attacks. Notice how people with COVID-19 vax-induced-myocarditis die during their sleep...
Deep sleep stages take place more during the first half of sleep, and REM more during the second half of sleep.
When you monitor sleep disorders, you have at your disposal a nocturnal ventilatory polygraphy which keeps track of your thoracic and abdominal movements, oxygen, heart rate, whether there is air flow in your nose or struggle of air flow in your throat. It also keeps track of your bed position while you sleep. It gives enough information to let you know if there's struggle while you sleep because there's too much snoring, because you stop breathing at night or because air flow is just reduced as the tongue collapses your throat due to reduced muscle tonicity as in this picture:
You can also use a polysomnography which adds a lot more parameters including electrical activity in the brain, then you know in which exact sleep phase the apnea is taking place.
When reading the heart monitor parameters during sleep and knowing which sleep phase the person is going through as you do with a polysomnography, if you stumble upon this you can assume the person is in heart health trouble:
It's higher heart rate variability of more than 20 beats per minute at night. The person was eventually diagnosed with sleep apnea. And the heart rate variability coincides with stress due to lack of oxygenation during an obstructive flow event. They called it sleep apnea dysautonomia, where the increased heart rate variability alerts the specialist to look for trouble. You go from a parasympathetic predominance with low heart rate while you sleep, to a sympathetic storm due to a micro-awakening while the person struggles to breathe and the heart just starts beating much faster. Definition of pathology: higher heart rate variability of more than 20 beats per minute at night.
As the thorax is trying to pump air inside and is fighting against the obstruction at the level of the throat, the heart gets compressed which stimulates a heart hormone (brain natriuretic peptide) and people with these problems get up at night to pee often since the hormone stimulates urination.
You can see sleep apnea in obese people, whenever the ear-throat-nose area is chronically inflamed, when there are anatomical anomalies where the amygdalas of the throat or the tongue are bigger than normal. You can also see it in people who had chronic upper airway problems when they were children, which made them chronic mouth breathers. That leads to developmental abnormalities of their facial features, which favors sleep apnea within the context of resistance of the upper airway. There's actually a syndrome that reveals their sleeping problems and manifest thus:
Patients suffering from "resistance of the upper airway syndrome" consult for what appears to be functional problems. They are referred to psychiatrists for management of insomnia, chronic fatigue, muscular disorders labelled as fibromyalgia, headaches, depressive syndromes... all of which represent the most common complaints. Sometimes, in young subjects, unexplained fainting is the revelatory sign of the syndrome [vasovagal syncope]. During this fainting spell, blood pressure tends to be sometimes very low, and the feet, hands and nose are cold. The patients concerned are generally younger than those with the classical obstructive sleep apnea, and complain of bruxism. Their wisdom teeth have often been removed because they were “impacted” between 18 and 25 years of age, and these patients have had orthodontic treatment as teenagers (often inappropriate, as they only consider the dental and not the maxillofacial problems).
On examination, they present with a deviated nasal septum, very enlarged lower choanae, and often report respiratory and/or nasal allergic problems. Above all, they have reduced maxillary dimensions, in the upper maxilla, the lower maxilla, or both (with orthodontic problems as a consequence) Guilleminault, Leger : Rev Mal respir 2005 ; 22 : 27-30
It's similar to a POTS (postural orthostatic tachycardia syndrome), but mechanical in its origin. You can see a picture of someone who had her anatomical issues corrected by a surgeon:
Sometimes physiotherapy is enough, but sleep disorders and its causes goes a bit beyond the subject of this thread.
In summary, heart rate variability is not necessarily always a sign of good health. It depends on which parameters they used for monitoring and what is the context. As it is, sleep disorder centers are the most advanced for monitoring. Companies like Oura and others have to go to them to test their technology and see if it compares to the scientific standard. One could argue that heart rate variability as a healthy marker of the smart vagus was never meant to indicate pathology as monitored above (which is a super zoomed version of monitoring). Nevertheless, the lack of consensus among scientists of what they're seeing, and the various interpretations, can give rise to confusion.