A Systematic Review and Meta-Analysis of Biological Sex Differences in Sleep Spindles and Slow Wave Activity in Adults with and without Insomnia

Mounting evidence shows sex-based differences in sleep experiences and outcomes, including the prevalence of insomnia disorder. However, the impact of biological sex on brain oscillations during sleep remains poorly understood, especially in the context of insomnia disorder. This is a notable gap, given that neurophysiological aspects of sleep are associated with brain health and overall sleep quality. We systematically reviewed and meta-analysed data from studies reporting spindle and slow wave activity in adults with and without insomnia disorder. We conducted systematic searches in PubMed, Embase, Scopus, and PsycInfo. Risk of bias was evaluated using the Cochrane Risk of Bias tool. Forty-three studies met our inclusion criteria, with thirteen studies of normal sleepers (N= 668) reporting sufficient data for random-effects meta-analyses. Compared with males, female normal sleepers had higher spindle density, sigma and delta power. Most studies recruited individuals with primary insomnia, and data pooling for insomnia and mixed groups was not possible due to insufficient statistical reporting. Moreover, group-by-sex interactions were limited, inconsistent, and varied across studies and sample characteristics. Further research is needed to explore sex-specific differences in sleep microarchitecture and their role in normal sleep and the manifestation of insomnia disorder.