Diagnostics · Objective monitoring · Beyond AHI

SleepImage: what it measures and why it matters

Written by Dr Ari Manuel, Consultant in Sleep & Respiratory Medicine · Last reviewed March 2026

Standard NHS sleep studies report one number: the apnoea-hypopnoea index (AHI). This counts breathing pauses per hour and has been the cornerstone of sleep medicine for decades. It is useful — but also incomplete. A growing body of evidence shows that AHI alone is a poor predictor of cardiovascular outcomes, symptom severity, and treatment response. Many patients with significant symptoms have a "normal" AHI. Many patients with a high AHI feel entirely well. The number, on its own, doesn't tell you what you actually need to know.

SleepImage was developed to address this gap. It is an FDA-cleared, CE-marked ambulatory monitoring platform worn as a ring during sleep, generating a set of biomarkers that reflect how the body's regulatory systems behave overnight — not just how often breathing pauses.

What SleepImage measures

Cardiopulmonary Coupling (CPC)

CPC is the centrepiece of the SleepImage platform. It measures the interaction between heart rate oscillations and respiratory effort during sleep — how well the cardiac and respiratory systems synchronise overnight. This synchronisation is a marker of sleep quality independent of breathing event frequency. SleepImage generates a spectrogram showing the proportion of sleep spent in three states:

• High Frequency Coupling (HFC) — stable, restorative sleep. This correlates with slow-wave sleep depth and physiological recovery. More is better.
• Low Frequency Coupling (LFC) — unstable, fragmented sleep. Elevated LFC is associated with insomnia, arousal, autonomic dysregulation, and poor daytime function — even without apnoeas.
• e-LFC (elevated Low Frequency Coupling) — a specific pattern associated with sleep-disordered breathing, particularly obstructive and central apnoeas.

The balance of these states across a night provides a detailed picture of sleep architecture that no questionnaire can replicate, and that an AHI-only report misses entirely.

Hypoxic Burden

Rather than simply counting desaturation events, SleepImage quantifies total overnight oxygen desaturation load — integrating both depth and duration of oxygen dips into a single measure. This "hypoxic burden" metric has been shown in large prospective studies to predict cardiovascular mortality more accurately than AHI alone (Azarbarzin et al., Eur Heart J 2019). It captures the cumulative cellular oxygen stress of a night's sleep in a way that event counting cannot.

Autonomic Indices and Heart Rate Variability

Heart rate variability (HRV) during sleep reflects the balance between sympathetic and parasympathetic nervous system activity. Chronically disturbed sleep shifts this balance toward sympathetic dominance — a state associated with increased cardiovascular risk, impaired immune function, metabolic dysregulation, and poor mood and cognition. SleepImage's autonomic indices quantify this across the night, providing an objective measure of physiological stress that daytime readings alone cannot capture.

Serial Monitoring

Because SleepImage is worn as a ring at home, it can be used for multiple consecutive nights — providing a far more representative picture than a single-night laboratory study. Results can be compared before and after treatment, tracking whether an intervention is producing real physiological change even before subjective symptoms improve.

Who benefits most

Patients with unexplained fatigue or cognitive symptoms and a normal AHI. A normal AHI does not mean normal sleep. CPC spectrograms frequently reveal highly fragmented sleep architecture in patients told their study was "fine." This is particularly common in women, in upper airway resistance syndrome (UARS), and in insomnia-driven arousal disorders.

Post-Covid and post-infection fatigue. Autonomic dysregulation is increasingly recognised as a core mechanism in Long Covid. SleepImage's autonomic and CPC indices can objectively quantify overnight dysregulation where questionnaires and standard sleep studies are normal — and allow treatment response to be tracked longitudinally.

Chronic pain and fibromyalgia. Slow-wave sleep is when descending pain inhibitory control is most active. CPC-derived measures of sleep stability provide an objective basis for sleep-targeted interventions in pain medicine, and can show whether those interventions are working.

Active adults and athletes. HRV and CPC during sleep are more sensitive measures of recovery quality than daytime HRV alone. SleepImage can explain persistently low morning readiness scores and identify whether the limiting factor is sleep architecture, overnight hypoxia, or autonomic imbalance.

CPAP users with residual symptoms. Effective CPAP normalises AHI. It does not always normalise sleep architecture. CPC monitoring can determine whether residual fatigue on CPAP is due to ongoing sleep fragmentation — and if so, what is driving it.

Cardiovascular and metabolic risk. Hypoxic burden and autonomic indices add prognostic information beyond standard sleep metrics in patients with hypertension, atrial fibrillation, heart failure, and type 2 diabetes — conditions where sleep quality materially affects outcomes.

How it works in practice

The ring is worn during sleep at home — no chest belts, no nasal cannulae, no laboratory visit required. It records photoplethysmographic data overnight, which is then uploaded and analysed. Results are reviewed by Consultant, contextualised within your full clinical history, and returned as a written report with plain-language interpretation and clear recommendations. The assessment is available as a standalone service or as part of a full consultation.