A small study of adults undergoing heart surgery found that patients with poorer sleep had lower heart rate variability (HRV) before their operation and during the first three days afterward.
The findings point to a measurable association between sleep quality and the way the heart’s rhythm changes from beat to beat. They do not show that poor sleep caused the lower HRV, or that HRV readings can currently predict recovery or complications.
What the study found
Tingting Lu and Jiudong Chen followed 95 adults undergoing heart surgery with cardiopulmonary bypass at Nanjing First Hospital between January 2023 and December 2024. The participants had an average age of about 70, and roughly half were men.
Sleep quality was assessed with the Pittsburgh Sleep Quality Index, an 18-item questionnaire scored from 0 to 21. The researchers used a score above 7 to classify a sleep disorder. Thirty-three participants fell into the poor-sleep group, while 62 were classified as better sleepers.
Compared with the better sleepers, the poor-sleep group had lower readings on five of eight HRV measures: HF power, SDNN, SDANN, pNN50 and RMSSD. These differences appeared the day before surgery and on postoperative days 1, 2 and 3.
The clearest association involved pNN50 on the first day after surgery. The average was 6.3% among poor sleepers and 10.3% among better sleepers, with a correlation of r = −0.78. In a regression analysis, each one-point increase in day-1 pNN50 was associated with about 84% lower odds of being in the poor-sleep group.
RMSSD, another measure related to beat-to-beat changes, averaged 27.4 milliseconds versus 33.5 milliseconds before surgery in poor and better sleepers, respectively. On day 1, the averages were 16.9 and 24.7 milliseconds. By day 3, they were 20.8 and 24.6 milliseconds, and the difference remained statistically significant.
HRV also changed after surgery in both groups
Heart rate variability fell in both groups on the first day after surgery and recovered only partly by day 3. The LF/HF ratio rose on day 1.
Other measures, including HF power, SDNN and SDANN, were also lower in poor sleepers at all four measurement points. For example, SDNN the day before surgery averaged 121.7 milliseconds in poor sleepers and 132.8 milliseconds in better sleepers.
Not every measure differed between the groups. LF power, the LF/HF ratio and SDNNI showed no significant group difference.
What heart rate variability can indicate
HRV describes the variation in time between successive heartbeats. Higher resting HRV is generally associated with more flexible regulation of the heart and greater rest-and-digest activity. Lower HRV can occur alongside stress, illness, fatigue or reduced vagal control, although a single reading does not establish a person’s overall health.
RMSSD reflects typical beat-to-beat changes, while pNN50 measures the percentage of consecutive beats that differ by more than 50 milliseconds. HF power is another marker linked with vagal activity. SDNN and SDANN are broader measures that reflect influences from both branches of the nervous system.
Why the results need caution
The study was observational. Sleep was assessed once, on postoperative day 3, but that result was used to classify all four HRV readings, including the presurgery measurement. As a result, the study cannot establish whether poor sleep contributed to lower HRV, whether lower HRV influenced sleep, or whether other factors affected both.
The research also did not track recovery, complications, hospital stay or other clinical outcomes. It did not report adjustments for medicines such as beta-blockers, sedatives or opioids, which can affect HRV and sleep.
The sample came from one hospital and included only 33 people in the poor-sleep group. The strongest associations should be interpreted particularly carefully: the source notes that the analysis selected the best-performing readings from 32 candidates and tested the resulting model in the same patients. Associations of this size may be smaller in larger studies.
There were also technical limitations. The recordings used three types of ECG devices, and the methods described five-minute recordings, although broader measures such as SDANN are normally based on 24-hour recordings. The source also reports inconsistencies in how the study design was described and in one reported SDNN result.
What patients should take from it
The findings support continued interest in sleep as part of care around major surgery, but they do not support using HRV as a standalone warning test or treatment guide. The researchers’ suggestion that HRV monitoring might help identify risk still requires testing in larger, multicenter studies that follow patient outcomes.
Anyone preparing for heart surgery should discuss persistent sleep problems, medication effects and postoperative concerns with their clinical team. This study does not establish that improving sleep will raise HRV or improve recovery.
AI tools were used to assist with the preparation of this article.
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