Sleep, dreams, nightmares, and sex-related differences: a narrative review

Objective: The aim of this study was to review the available findings on sex-related differences for sleep disorders, dreams and nightmares. Materials and methods: We explored the PubMed, EMBASE and Google Scholar electronic databases, with regards to the searching terms 'sleep', 'dreams', and 'nightmares' associated with 'sex' and/or 'gender'. Moreover, other supplementary terms for the searching strategy were 'chronobiology', and 'circadian rhythm'. Due to the relative paucity of studies including separate analysis by sex, and especially to their wide heterogeneity, we decided to proceed with a narrative review, highlighting the sex-related findings of each topic into apposite boxes. Results: On one hand, sleep disorders seem to be more frequent in women. On the other hand, sex-related differences exist for either dreams or nightmares. As for the former, differences make reference to dream content (men: physical aggression, women family themes), self-reported perspective (men dream in third person, women in first person), dream sharing (more frequent in women), lucid dreaming (women more realistic, men more controlled), and daydreaming (young men more frequently have sexual themes). Nightmares are more frequent in women too, and they are often associated with sleep disorders and even with psychiatric disorders, such as depression and/or anxiety. In women, a strong association has been shown between nightmares and evening circadian preference. Conclusions: For many years, and for many reasons, laboratory experiments have been conducted mainly, if not exclusively, on male animals. Thus, a novel effort towards a new governance of scientific and research activities with a gender-specific perspective has been claimed for all areas of medicine, and more research on sex-differences is strongly needed also on this topic.