Lead
Why do women experience chronic insomnia more often and more severely than men? A new study published in Sleep suggests part of the answer lies in subtle differences in the electrical rhythms that characterize deep sleep. Researchers led by Nyissa A. Walsh at Concordia University in Montreal found that chronic insomnia disorder and biological sex each independently alter the brain’s NREM sleep oscillations, and that the combination of being female and having insomnia produces a distinctive neurophysiological profile that may help explain the higher burden of insomnia in women.
The study of 222 adults between the ages of 18 and 82 reveals that insomnia primarily blunts two key NREM oscillations — sleep spindles and slow oscillations — regardless of sex, while sex itself accounts for differences in slow-wave activity. Female participants with chronic insomnia showed the most pronounced reductions in spindle density and sigma power, a pattern their authors argue could underlie greater insomnia severity in women.
The NREM oscillations
The brain does not simply go quiet during sleep. During non-rapid eye movement (NREM) sleep, it generates a series of distinctive electrical oscillations that are critical for memory consolidation and the restorative functions of rest. The two most studied are sleep spindles — brief bursts of 11-16 Hz activity originating in the thalamus — and slow oscillations (SOs), slow waves that sweep across the cortex roughly once per second. Together with slow-wave activity (SWA), the power in the delta frequency band, these oscillations form the backbone of deep NREM sleep.
Aging is known to reduce all of these oscillations, but the independent contributions of sex and chronic insomnia disorder have been harder to isolate. Walsh and colleagues designed their study to disentangle these factors by enrolling a large sample spanning early adulthood to old age and controlling for age as a covariate in all analyses.
After a habituation night, participants underwent one night of polysomnography. The team analyzed spindle density (spindles per minute), slow oscillation density, relative sigma power (the spectral power in the spindle frequency range), and slow-wave activity across frontal and central electrode sites.
The results showed that age, insomnia disorder, and sex each made unique, independent contributions to the NREM oscillatory landscape.
The sex difference
Even among healthy sleepers, men and women showed different NREM oscillation profiles. Women overall had higher slow-wave activity than men, a finding consistent with prior literature. But the more striking sex differences emerged when the researchers looked within the insomnia group.
Female participants with chronic insomnia had lower sigma power than their male counterparts with insomnia. Sigma power — the spectral energy in the sleep spindle frequency band — reflects the brain’s capacity to generate spindles, which are thought to play a protective role in stabilizing sleep against disruptive stimuli. Lower sigma power in women with insomnia suggests a reduced ability to maintain sleep continuity in the face of internal or external disturbances.
The female insomnia group also had the highest insomnia severity scores of any subgroup. When the researchers compared female insomnia participants to female healthy sleepers, the insomnia group showed significantly reduced spindle density and slow oscillation density. The same pattern held for males — male insomnia participants had lower spindle and SO density than male healthy sleepers — but the female insomnia group carried the additional deficit of reduced sigma power relative to both female healthy sleepers and male insomnia participants.
The insomnia deficit
Across both sexes, chronic insomnia disorder was associated with a clear neurophysiological signature: lower spindle density and lower slow oscillation density compared to healthy sleepers. This deficit was present independently of age and sex, suggesting that insomnia itself dampens the brain’s ability to generate these key NREM oscillations.
Spindles and slow oscillations are not merely epiphenomena of sleep. Spindles are known to gate sensory information during sleep, helping the brain remain asleep despite environmental noise. Slow oscillations coordinate the timing of spindles and help drive the glymphatic clearance of metabolic waste from brain tissue. A reduction in both oscillations may represent a dual hit — diminished sleep protection and less efficient brain maintenance during the night.
The study’s design, which controlled for age, is particularly informative. Because both spindle density and SO density decline with age, earlier studies that did not account for age may have conflated age-related changes with insomnia-related changes. Walsh and colleagues show that insomnia contributes an additional, age-independent reduction in these oscillations.
Why it matters
Chronic insomnia affects an estimated 10-15% of adults worldwide, and the prevalence is roughly 1.5 times higher in women than in men. The mechanisms driving this sex disparity have remained poorly understood. The current study points to a concrete neurophysiological pathway: women with chronic insomnia have a distinct pattern of reduced sigma power and diminished spindle and slow oscillation density that may both reflect and contribute to their higher symptom burden.
These findings have potential clinical implications. If reduced spindle density and sigma power are mechanistic drivers of insomnia severity, then interventions that enhance these oscillations — such as acoustic stimulation during slow-wave sleep, cognitive training targeting thalamocortical function, or closed-loop neuromodulation — could be tested with sex-specific endpoints. For female patients, treatments aimed at boosting sigma power may be particularly relevant.
The study also reinforces the importance of sex as a biological variable in sleep research. Including sex as a covariate is not enough if sex-specific effects are the question. Walsh and colleagues demonstrate that the interaction between sex and insomnia produces a neurophysiological profile that neither factor alone can predict.
Limits
The study used a single night of polysomnography, which may not capture night-to-night variability in sleep oscillations. The sample, while relatively large, was not evenly balanced by sex across groups — 71% of the insomnia group was female — which may limit the power to detect sex-by-group interactions. The authors note that their findings are correlational and cannot establish whether the oscillation deficits precede insomnia or result from it. Future longitudinal studies tracking these oscillations over time will be needed to establish causality.
Bottom line
Chronic insomnia and biological sex independently reshape the brain’s NREM sleep oscillations in ways that go beyond the effects of aging. Women with insomnia show a particularly vulnerable neurophysiological profile — lower sigma power, fewer spindles, and fewer slow oscillations — that may explain why they experience insomnia more frequently and more severely than men. Understanding these sex-specific mechanisms opens the door to more targeted, personalized treatments for chronic insomnia.
Source
Walsh NA, et al. Beyond Aging, Sex and Insomnia Disorder Shape NREM Brain Oscillations. Sleep. 2026. DOI: 10.1093/sleep/zsag192. PMID: 42454951.