Sex Differences in the Transitional Period between Slow-Wave Sleep and REM Sleep: A Novel Metric for Sleep Quality and Neuropsychiatric Disorders

Sleep is an important biological process, and it is well-documented that sleep disturbances often precede neuropsychiatric disorders. Sleep has traditionally been divided into two basic stages, non-REM and REM sleep. These two stages are often considered to have distinct functions and are usually studied independently. However, an additional sleep stage, the transitional period between non-REM and REM sleep, has been described but not well investigated. While sex differences in the other stages of sleep have been documented and are deemed clinically relevant, no study has investigated sex differences in this transitional period. In the present study, this period is termed transition sleep is directly compared between male and female rats for the first time. Eight male and eight female adult rats were fitted with sleep-recording implants, and their sleep-wake activity was recorded every four days for two weeks. The results indicate that female rats spend more time in transition sleep and have a higher frequency of transition sleep episodes. Additionally, regression models of the percentage of time spent in each sleep stage revealed that the different sleep stages had larger effects on one another in the female rats compared to the males. This indicates that, for the females, the different sleep stages were more interdependent, pointing to a difference in the biological mechanisms that are involved in regulating transitions between sleep stages. These findings provide novel insight into sex differences in sleep and a new approach for investigating the link between sleep disturbances and psychiatric illness.