The new study, published in Physiological Reports, was led by Marion Faivre, Jérôme Thireau and François Bughin at Montpellier University Hospital, with colleagues across French institutions. It is, according to the authors, the first investigation to examine how sleep apnea affects both nighttime and daytime markers of cardiovascular autonomic neuropathy—a dangerous dysfunction of the nerves that regulate heart rhythm and blood vessel tone—specifically in people with type 1 diabetes. Cardiovascular autonomic neuropathy is considered a key driver of cardiovascular death in diabetes, and the American Diabetes Association already recommends systematic screening for it. Understanding what pushes patients toward that condition is therefore a matter of real clinical urgency.

The rationale for the study stems from an increasingly troubling picture. Even among patients who achieve strict glycemic control, cardiovascular risk in type 1 diabetes remains about twice as high as in the general population, meaning that blood sugar alone cannot explain the excess. In a large French nationwide cohort published in 2023, moderate to severe obstructive sleep apnea—defined by an apnea-hypopnea index of 15 or more events per hour of sleep—was found in up to 40 percent of people with type 1 diabetes, and those patients showed a nearly fourfold increased risk of macrovascular complications. Sleep apnea has long been known to stress the cardiovascular system in people without diabetes, but no one had systematically compared autonomic function during sleep and wakefulness between type 1 diabetes patients with and without significant apnea.

To fill that gap, the team recruited 21 adults with type 1 diabetes, aged 18 to 60, each with at least five years of disease duration, from the endocrinology and diabetology department at Montpellier University Hospital. Patients on continuous positive airway pressure therapy, those taking drugs that interfere with heart rate variability such as beta-blockers, and people with pacemakers or neuromuscular disease were excluded. Each participant underwent a comprehensive daytime assessment of autonomic function followed by a full overnight polysomnography in the hospital sleep laboratory, with fasting blood and urine samples collected the next morning. The study was approved by French ethics review boards and registered as NCT03605329.

The polysomnography recorded brain waves, eye movements, muscle tone, airflow and respiratory effort according to American Academy of Sleep Medicine guidelines, allowing researchers to score each patient’s apnea-hypopnea index, or AHI—the number of breathing interruptions per hour. Fourteen patients had an AHI below 15 per hour, while seven had moderate to severe sleep apnea with an AHI of 15 or more. As expected, the apnea group had dramatically more breathing events both during REM sleep (39.4 versus 12.9 events per hour) and during non-REM sleep (27.6 versus 5.4 per hour), along with more frequent oxygen desaturations and a higher arousal index.

The centerpiece of the investigation was heart rate variability analysis, a well-validated technique that decomposes the tiny beat-to-beat fluctuations of heart rhythm into frequency bands. High-frequency power, typically expressed in normalized units, reflects parasympathetic—vagal—activity, while the low-frequency to high-frequency ratio gauges the balance between sympathetic “fight-or-flight” drive and vagal braking. The researchers manually extracted five-minute segments of heartbeat data from each sleep stage, carefully excluding periods containing arousals, and compared them with a segment recorded during quiet wakefulness just before lights-out. Ectopic beats and artifacts were removed before spectral analysis to avoid contaminating the sympathetic markers.

The results were striking. During REM sleep, patients with an AHI of 15 or more showed a high-frequency power of just 12.1 normalized units on average, compared with 25.3 in patients with milder apnea—a statistically significant drop indicating markedly reduced vagal, or parasympathetic, control of the heart. Their average heart rate during REM was also substantially faster: 73 beats per minute versus 64. Across the entire cohort, the low-frequency to high-frequency ratio during REM correlated positively with apnea severity, and REM sleep proved far more susceptible to autonomic disturbance than deep N3 sleep, where vagal tone normally dominates. In other words, the more severe the apnea, the more the night-time balance of the autonomic nervous system tipped away from the heart-protective parasympathetic branch.

Biochemical evidence told the same story. Morning urine samples, analyzed by high-performance liquid chromatography and normalized to creatinine, revealed a significantly higher norepinephrine-to-epinephrine ratio in the sleep apnea group—16.2 versus 3.5. Because urinary norepinephrine relative to epinephrine reflects peripheral sympathetic nervous system output, this finding points to sustained sympathetic activation in patients whose airways collapse repeatedly during the night. A trend toward higher norepinephrine excretion reinforced the picture of an over-stressed sympathetic system.

Interestingly, the study did not find a higher proportion of diagnosed cardiovascular autonomic neuropathy among patients with significant apnea. Standardized daytime cardiovascular reflex tests—including heart rate responses to deep breathing, standing and the Valsalva maneuver, plus blood pressure responses to standing and sustained handgrip—showed no significant group differences, nor did wakefulness heart rate variability or sudomotor function measured with Sudoscan electrodes. However, one telling correlation emerged: the drop in systolic blood pressure on standing, a marker of autonomic dysregulation, correlated with apnea severity across all patients (r = 0.58, p = 0.01). This suggests that daytime consequences may become measurable only in more severe or longer-standing cases, consistent with prior research showing that nocturnal heart rate variability is impaired earlier and more strongly than daytime measures in sleep apnea.

The authors are candid about their study’s limitations. The sample was small and unbalanced—seven versus fourteen patients—and recruitment was cut short by the COVID-19 pandemic. The apnea group tended to be older, although a large meta-analysis suggests age has little effect on the specific vagal marker that differed between groups. Curiously, the milder apnea group had the higher HbA1c, meaning poorer glycemic control may have masked, rather than inflated, the apnea effect on heart rate variability. The cross-sectional design also prevents any claim that sleep apnea causes autonomic neuropathy; it can only show that the two are linked during sleep.

Even so, the convergent findings—from spectral heart rate variability, from urinary catecholamines and from blood pressure reflexes—paint a coherent picture. Sleep apnea in type 1 diabetes is associated with a loss of nocturnal vagal protection and a surge of sympathetic activity precisely during REM sleep, a stage in which the heart is already inherently vulnerable. Animal models and human studies have both shown that intermittent airway obstruction and the accompanying oxygen dips can generate hypertension and chronic sympathetic overactivity, and modern functional MRI studies have even identified structural and functional changes in the brain’s autonomic networks of apnea patients. In a population whose autonomic nerves are already threatened by diabetes, this nightly assault could plausibly accelerate the march toward cardiovascular autonomic neuropathy and, ultimately, cardiovascular events.

The clinical implications are considerable. Sleep apnea affects an estimated 16.7 percent of people with type 1 diabetes and roughly one in four of those with autonomic neuropathy, yet it often goes undiagnosed because patients paradoxically lack daytime sleepiness. The French cohort data linking moderate apnea to a nearly fourfold rise in macrovascular complications make a strong case that clinicians caring for type 1 diabetes patients should consider routine screening for sleep-disordered breathing. If future longitudinal studies confirm that treating apnea—typically with nightly positive airway pressure—preserves vagal tone during sleep and slows the development of autonomic neuropathy, a simple and treatable sleep condition could become a modifiable lever in the fight against the leading killer of people with type 1 diabetes. The Montpellier team calls for exactly such studies, including mechanistic work examining how the hypoxic burden of apnea interacts with autonomic nerve damage to amplify cardiovascular risk.