Women have higher density of sleep spindles and greater power in both sigma and delta frequency bands during sleep compared with men, according to a systematic review and meta-analysis published in the Journal of Sleep Research. The finding holds for healthy adults but could not be confirmed in people with insomnia, where data remain too sparse for pooled analysis.
What they found
The review identified 43 studies meeting inclusion criteria. Of these, 13 studies of healthy sleepers (668 participants) provided sufficient data for random-effects meta-analysis.
In healthy adults, women compared with men showed:
- Higher spindle density, more sleep spindles per minute of non-REM sleep.
- Higher sigma power, the EEG frequency band (12-16 Hz) dominated by spindle activity.
- Higher delta power, the slow wave frequency band (0.5-4 Hz) associated with deep sleep.
These differences were consistent across studies and independent of age.
For studies of insomnia, the picture was less clear. Most recruited individuals with primary insomnia, but inconsistent statistical reporting and limited sex-by-group interaction analyses prevented meta-analysis. Some studies reported sex differences in sleep microarchitecture among people with insomnia, while others did not.
Why it matters
Sleep spindles and slow waves are not just passive markers of sleep depth. Spindles are linked to memory consolidation, IQ, and protection against external disruption during sleep. Slow wave activity reflects synaptic homeostasis and glymphatic clearance. That these oscillatory features differ between sexes has implications for how sleep research is designed and interpreted.
Many sleep studies control for sex statistically or match groups by sex, but few treat sex as a primary variable of interest. If women and men have fundamentally different sleep EEG signatures by default, then normative ranges, diagnostic thresholds, and even treatment targets may need sex-specific calibration. This is especially relevant for insomnia, which affects women at roughly double the rate of men.
The authors also note that hormonal influences — estrogen and progesterone modulate GABAergic and glutamatergic neurotransmission — may drive at least part of the difference, though the included studies did not directly test hormonal mechanisms.
Limits
The meta-analysis pooled data only from healthy sleepers. The insomnia arm of the review was descriptive only, as too few studies reported the statistics needed for quantitative synthesis. Most studies were cross-sectional, so causal direction cannot be established. The authors highlight the need for future studies to report sex-stratified data and to examine how sex differences in sleep microarchitecture relate to the pathophysiology of insomnia.
Bottom line
Sleep spindles and slow wave activity differ by sex in healthy adults, with women showing consistently higher values. The finding underscores the need for sex-aware reference standards in sleep EEG research and clinical practice.
Source
Nyissa A. Walsh et al. “A Systematic Review and Meta-Analysis of Biological Sex Differences in Sleep Spindles and Slow Wave Activity in Adults With and Without Insomnia.” Journal of Sleep Research, June 22, 2026. DOI: 10.1111/jsr.70388