Looking Differently at Sleep

What do we know about sleep in brief?

Sleep as been a subject of study for many years now, from a physiology and psychology point of views. We have a rough idea what is happening during sleep in our brain and have some hints of our psyche’s dynamic.

Sleep is understood from a brain activity perspective as a succession of stages where we go into deep sleep, light sleep, dream states and so on. We ‘fall’ asleep at he beginning of our journey to dream lands and awaken our mind, brain and organs in an apparently organised succession of events according to laws that we still have no clue about. So to tell the truth, we have very little knowledge about why we sleep and what is happening during this time.

Sleep is the best medicine as our body and mind are supposed to rest and recover from daily stress (WHO report on sleep). It is not always the case as we experience more and more often bad quality of sleep, meaning that we feel tired when waking up or dozing during the day. But how to objectively measure this quality of sleep is still in a research phase, despite many metrics being published and scientifically studied. Indeed, it is often hard to correlate physiological phenomena with subjective quality of sleep as research points to different answers to this question [1].

The picture is not so dark as we clearly understand that lack of recovering sleep has a large impact on our life, both physically and psychologically [2]. However the quantification of what is a good recovering sleep is still largely missing and is mainly due to our lack of understanding of what sleep is all about.

Sleep physiology has been and still assessed by the use of the sleep staging concept (see below this post), whereby sleep is classified into several degrees of depths, thus clearly referring to different levels at which our conscious mind is active/present or no. Sleep clinicians refers to deep sleep, dream sleep and 3 degrees of lightest sleep. However, one may rightfully ask the question why there would be such a simple sleep organisation of our mind state? It feels like we are indeed at the very beginning of understanding how the mind function during sleep, and thus by extension during the awaken state.

The relationship between heart rhythm and brain activity is a research area since a while now and researchers start to have a clue that this relationship is very important regarding sleep quality [3,4,5]. Moreover, it has been a long term study of the relationship between the way we breath and our heart rhythm during sleep [6] showing some coordination of the two functions depending on the sleep ‘depth’, state of our consciousness or even apnea events [7,8].

The conclusion of this brief introduction about sleep and the way to quantify it is that the heart, brain and breathing interactions are playing a big role into the structure of our sleep and probably hints at how we can measure a good sleep.

SATHeart approach to sleep

In the graph below, we can observe a sleep pattern from a person with the coordigram (top) showing the depth of sleep and the manifestation of the three humours (bottom). The depth of sleep is color-coded: the blue zones indicates a high level of heart-brain coordination with slow regular breathing, while the red zones indicate a low coordination level with a higher breathing rate. It is thus understood that the blue zones corresponds to deeper sleep, while the red zones to a lighter sleep. The red zones correspond to a state of stress while the blue ones to a state of relaxation. The red and blue zones alternates in cycles of 10 to 20-30 minutes minutes. The black curve indicates the state of average ‘stress’, increasing in the red zones and decreasing in the blue ones. SATHeart takes these cycles into account to derive indices of sleep quality. These indices have few relationship with sleep duration showing that quality is not necessarily linked with quantity.

Additionally, SATHeart developed a unique technique to assess sleep using the following three Energetic Principles (S.A.T.): 1) Stability (Blue), 2) Activity (Green) and 3) Transformation (Red). These three aspects are quantified via a detailed analysis of the blood pulse wave shape and its rhythm. These three energetic principles also reflect conscious and emotional processes, thus forming a complete holistic approach to human health. These three informations are displayed below the colour map in the forms of ellipses of the three colours and different size according to their value. The basic principle is that a good night sleep should be as stable as possible and minimum activity. The transformative aspect is linked with how fast the pulse wave shape and/or the rhythm is changing.

The coordigram shown below is radically different from the one shown above with few red zones after falling asleep and mostly dominated with blue zones along the night showing a stable sleep.

Yet an other type of sleep profile is shown below. This person had a great deal falling asleep for more than one hour, then finally started to relax. This night sleep was also shorter than the two previous examples. The stress curve shows a nice dip at about two hours after bed time and is more or less stabilised after three hours. The Transformative aspect is dominating the beginning of the night and fade away slowly, similarly to the Activity aspect. The Stability is showing up sparsely from the middle of the night.

The last sleep profile is archetypal of a good night sleep with alternating periods of each energetic principle which themselves correspond to the self-healing of our body and mind. The red zones are very scarce and the stress curve shows a nice and smooth behaviour with easily recognisable periods of deep sleep and lighter sleep.

The energetic principles are used complimentary to the coordigram as they measure deeper structure of sleep physiology and mind processes using the blood pulse wave shape in addition to the heart rhythm used in the coordigram. These three energies are used at the backstage of SATHeart cloud analytics to quantify the health state of the person in a more holistic way: heart function, nervous system function, breathing function, vascular tree function, conciousness processes.

Comparing with Hypnogram

We have compared our approach with the gold standard of sleep assessment which is the hypnogram. The hypnogram tells us how deep we are sleeping and for how long in the different stages. The classification of sleep stages is thus done essentially looking at physiological or body responses and is called the macrostructure of sleep. There are 5 to 6 stages of sleep: WAKE, NR1, NR2, NR3/NR4 and REM. NR3 and NR4 stages have been merges into one stage recently by the American Academy of Sleep Medicine.

Your night sleep starts with a little dip into the darkness, still half-awake half asleep. Voices are still resonating around, and you could even still answer questions if any, but the body is less and less responding to your conscious will. This is the toboggan of sleep from wake (WAKE) to light sleep called (NR1 – Non REM stage 1) as shown in the figure below, which illustrates a person’s night sleep. The fall continues quite rapidly to deeper stages NR2 and NR3, until it reaches the bottom NR4, the deep sleep mode or also called slow wave sleep because the brain waves are very low frequencies. This tobogganing last between 20 minutes to 1 hour if all goes well.

The figure above shows an example of our night sleep stress metric together with sleep staging. The macrostructure (hypnogram sleep stages) is shown in the plot below the coordigram and is color coded as follows: NR4 (black dots), NR3-NR1 (increasing lighter grey dots), REM as orange dots and WAKE as red dots. For ease of interpreting the correspondence between our stress metric and the sleep stages, we have color coded our metric with the sleep stages. We can see that our stress index follows quite neatly the sleep stages and that the deep sleep stage NR4 corresponds to low stress index, REM stages to medium stress and WAKE stage to high stress.


SATHeart blood pulse wave analysis is used complimentarily to heart rate and heart rate variability. The takeaway message from SATHeart is that there is a lot more to say with deeper knowledge of the blood pulse wave shape and its interpretation in terms of the three characteristics of Stability, Regularity and Transformation. Also, SATHeart quantify the relationship between brain, heart and breathing which has been proven to be also very valuable information when we assess quality of sleep [7,8].

Sleep is understood as the time when body recovery take place and when eventually the daily stress is being compensated to ‘recharge’ our batteries. A simple analysis of the minimum heart rate and the maximum parasympathetic activity are not enough to account for a full night sleep where many recovery phenomena occurs from cells, to hormonal tissues, vessels’ endothelium, heart and brain functions and conscious state dynamics. Thus, SATHeart is providing a holistic view of most of these phenomena by encompassing the body and mind dynamics.

The SATHeart metrics are not only unique and insightful, but have a very practical implication in terms of digital health technology: Our stress metric allows us to determine the quality of sleep without the need to perform a full night sleep hypnogram as most of the information about the quality of sleep is captured in the first part of the night. Additionally, it is well known that the establishment of an hypnogram depends on the sleep clinician and is thus subjective [9].

Your SATHeart Team


  1. Harvey, Allison G., et al. “The subjective meaning of sleep quality: a comparison of individuals with and without insomnia.” Sleep 31.3 (2008): 383-393.
  2. Ustinov, Yuriy, et al. “Association between report of insomnia and daytime functioning.” Sleep Medicine 11.1 (2010): 65-68.
  3. Wei, Yishul, et al. “I keep a close watch on this heart of mine: increased interoception in insomnia.” Sleep 39.12 (2016): 2113-2124.
  4. Mensen, Armand, et al. “The occurrence of individual slow waves in sleep is predicted by heart rate.” Scientific reports 6 (2016): 29671.
  5. Chouchou, Florian, and Martin Desseilles. “Heart rate variability: a tool to explore the sleeping brain?.” Frontiers in neuroscience 8 (2014): 402.
  6. Penzel, Thomas, et al. “Modulations of heart rate, ECG, and cardio-respiratory coupling observed in polysomnography.” Frontiers in physiology 7 (2016): 460.
  7. Niizeki, Kyuichi, and Tadashi Saitoh. “Association between phase coupling of respiratory sinus arrhythmia and slow wave brain activity during sleep.” Frontiers in physiology 9 (2018): 1338.
  8. Riedl, Maik, et al. “Cardio-respiratory coordination increases during sleep apnea.” PloS one 9.4 (2014): e93866.
  9. Danker‐hopfe, Heidi, et al. “Interrater reliability for sleep scoring according to the Rechtschaffen & Kales and the new AASM standard.” Journal of sleep research 18.1 (2009): 74-84.

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