Sleep spindle duration and frequency show significant nonlinear relationships with the first-night effect, with specific thresholds (inflection point around 0.9 s duration and ~13.3 Hz frequency) essential for modulating adaptation to new sleep environments.
Key Findings
Results
Night 1 of polysomnographic recording showed significantly worse macrostructural sleep compared to Night 2, consistent with the first-night effect.
92 healthy participants underwent polysomnographic recordings on two consecutive nights
Night 1 showed significantly lower total sleep time (TST) and lower sleep efficiency (SE) than Night 2
Night 1 showed higher wake after sleep onset (WASO) than Night 2
Night 1 showed longer REM latency (RL) than Night 2
Sleep efficiency change (ΔSE) between nights was used as the primary measure of first-night effect
Results
Sleep spindle characteristics as group-level measures remained stable between Night 1 and Night 2.
Paired Wilcoxon tests were used to compare night-to-night differences in spindle amplitude, duration, frequency, and density
No significant overall differences in spindle characteristics (amplitude, duration, frequency, density) were found between the two nights
Despite macrostructural sleep differences, spindle microstructure at the group level did not change significantly
Results
GAM analysis revealed a significant nonlinear relationship between spindle duration on Night 1 and the magnitude of the first-night effect.
Hierarchical Generalized Additive Models (GAMs) were used to evaluate nonlinear relationships with ΔSE
An exacerbated first-night effect was specifically linked to shorter spindle durations on Night 1
An inflection point was identified at approximately 0.9 seconds for spindle duration
Spindle durations below ~0.9 s were associated with worse sleep adaptation (greater ΔSE)
Results
GAM analysis revealed a significant nonlinear relationship between spindle frequency on Night 1 and the magnitude of the first-night effect.
Spindle frequencies that deviated from approximately 13.3 Hz were associated with an exacerbated first-night effect
Both higher and lower spindle frequencies relative to ~13.3 Hz were associated with worse sleep adaptation
This nonlinear (non-monotonic) relationship was identified specifically for Night 1 spindle frequency
Results
Relative slow oscillation (SO) power during slow-wave sleep was higher on Night 1, while delta power was reduced.
EEG power spectral density (PSD) was analyzed in the slow oscillation and delta bands during slow-wave sleep (SWS)
Relative slow oscillation power during SWS was notably higher on Night 1 compared to Night 2
Relative delta power during SWS was reduced on Night 1 compared to Night 2
These opposing shifts in SO and delta power suggest altered sleep microstructure during initial laboratory sleep
Discussion
Spindle parameters are proposed as neurophysiological indicators of an individual's ability to adapt to environmental sleep changes.
Specific thresholds of spindle duration (~0.9 s) and frequency (~13.3 Hz) were identified as essential for modulating adaptation to new sleep environments
The authors suggest spindle parameters may serve as targets for interventions to support sleep adaptation
The study frames sleep spindles as playing a role in the neurophysiological mechanisms underlying the first-night effect
What This Means
This research suggests that when people sleep in a new environment (such as a sleep laboratory), their brain activity during sleep shows specific patterns that relate to how well they adapt. The study recorded brain activity during sleep in 92 healthy adults over two consecutive nights and found that the first night was generally worse — people slept less efficiently, woke up more during the night, and took longer to reach REM sleep. However, when the researchers looked at fine-grained brain wave patterns, they found that the overall averages of sleep spindles (brief bursts of brain activity that occur during sleep) were similar across both nights.
The more revealing finding came from a statistical analysis that looked at nonlinear relationships: individuals who had shorter sleep spindles (under about 0.9 seconds) or whose spindle speed (frequency) deviated from approximately 13.3 Hz on the first night tended to show a worse first-night effect. Additionally, the balance of slow brain wave activity shifted on the first night, with more very slow oscillations and less delta activity during deep sleep. These patterns together suggest that sleep spindles may reflect the brain's ability to adjust to unfamiliar sleeping conditions.
This research matters because the first-night effect is a common challenge in sleep research and clinical sleep testing — patients and research participants often sleep poorly on their first night in a lab, which can make results harder to interpret. This study suggests that measuring sleep spindle characteristics could help identify individuals who are particularly susceptible to disrupted sleep in new environments, and could potentially guide the development of targeted strategies to help people adapt more quickly to new sleeping situations.
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Yang H, Chen Y, Xu Y, Su W, Paudel D, Fang L, et al.. (2026). Sleep microstructure correlates of the first-night effect: sleep spindles and slow-wave activity in healthy adults.. Sleep & breathing = Schlaf & Atmung. https://doi.org/10.1007/s11325-026-03732-w