The Royal Navy discharged five sailors last year for sleepwalking, a freedom of information request has revealed.
The Ministry of Defence's navy command secretariat said armed forces' medical rules are being updated so anyone who suffers from "sleep terrors" will be disqualified from joining the service, the Press Association reported.
Those with sleep apnoea – a disorder in which breathing is interrupted during sleep – are currently barred from joining the service.
In its FOI response, the Ministry of Defence said: "Sleepwalking is a bar to entry to the Royal Navy. Whilst this is not presently enshrined in MoD policy ... sleepwalking beyond the age of 13 is seen as a permanent medical disqualification. Policy updates to that effect are currently in the process of being cleared for publication.
"The permanently disqualifying nature of sleep terrors will similarly be reflected in the pending ... policy update."
No details were provided about how the five sailors' sleepwalking was discovered.
The MoD also revealed that since 2011, 12 service personnel have been rated as having a "medically limited deployability" (MLD) for a "sleep-related disorder".
Individuals with MLDs must undergo a risk assessment before each deployment.