Identified neurons B4/B5 function as sensory neurons, motor neurons, and interneurons in Aplysia.

Failure of individuals to meet the required standards of military service can negatively impact operational capability and lead to a less effective workforce. A common, undesirable, and undetected feature that jeopardizes the overall success and health of service personnel during military service is sleep disturbance [, ]. Despite a growing awareness of the importance of sleep for optimal functioning, particularly mood and mental health outcomes [], poor sleep quality and chronic sleep restriction continues to be reported [, ] and is considered a major health concern for many military organisations worldwide [, ]. Service personnel are frequently presented with unique operational challenges integral to military service, including operating within a highly regulated environment, sleeping in multi-occupancy rooms, routine travel across multiple time zones, and engaging in frequent high-risk tasks (e.g. simulated combat training and deployed operations). Additionally, insufficient sleep due to disrupted circadian sleep–wake cycles from chronic and irregular sleep–wake schedules, unfavourable sleeping environments and worries about family during prolonged training and deployments are some extant precipitating factors to poor sleep behaviours and the development of more severe sleep issues and associated comorbidities []. Military service personnel are at greater risk of chronic sleep disturbances relative to the general population [, ]. For instance, it is said that ~40% of the general population sleep on average less than the minimum recommendations of 7 hours per night [], compared with ~80% of service personnel reporting chronic short-sleep duration of <6 hours per night, and ~60% achieving <5 hours a night []. Poor sleep quality and insomnia-like symptoms, which are frequently comorbid with other sleep, physiological, and psychological issues, are common in service personnel, including but not limited to, breathing-related sleep disorders (e.g. obstructive sleep apnea [OSA]), circadian rhythm disorders, obesity, menstrual dysfunction in women [], metabolic and cardiovascular disorders, depression, anxiety, substance abuse, and suicidality []. Based on large observational studies, conducted mainly in US Army soldiers, most commonly reported sleep disorders include insomnia and breathing-related sleep disorders (similar to the general population), followed by parasomnias and circadian rhythm disorders, particularly those operating on rotating shift patterns [, , ]. Furthermore, clinically relevant and probable insomnia have been reported across multiple cohorts of British Army recruits [, ], UK veterans [], and active-duty US personnel, with symptoms frequently persisting beyond military service and commonly occurring alongside other adverse health outcomes []. The prevalence of sleep disturbances in UK service personnel is currently unknown; however, there is evidence of a link between sleep issues and adverse mental health outcomes in military personnel from other nations. In a US Army cohort, Mysliwiec, et al. [] reported a ~40% prevalence of comorbid insomnia with sleep apnoea (COMISA) and demonstrated a higher proportion of individuals with COMISA meeting clinical criteria for depression (71%) and PTSD (60%) compared to those with OSA alone (11%). Byrne, et al. [] observed a 3-5 times greater prevalence of suicidal ideation in US service personnel with clinical and subthreshold insomnia compared with those without insomnia. Additionally, Gehrman, et al. [] reported a 2-3 times greater risk of new-onset PTSD in US service personnel with a pre-deployment insomnia diagnosis compared with those without. Personnel with the most severe phenotypes (i.e. pre-existing insomnia and short-sleep duration) were at more than three times the risk of being diagnosed with new-onset PTSD.