The Royal Navy discharged five sailors last year for sleepwalking, it has been disclosed.
Anyone suffering from the disorder is also barred from joining the senior service, a Freedom of Information (FOI) request also showed.
The Armed Forces' medical rules are being updated to make the entry bar official and also to disqualify those who suffer from "sleep terrors", the Ministry of Defence's Navy Command Secretariat said.
Medical regulations already block sufferers of sleep apnoea, a condition where people stop breathing while they are asleep, the MoD added in its reply to the FOI.
It 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."
It added: The permanently disqualifying nature of sleep terrors will similarly be reflected in the pending ... policy update."
No details were given about how the five sailors' sleepwalking was discovered.
The FOI also disclosed that 12 service personnel have been rated as having a "medically limited deployability" (MLD) for a "sleep-related disorder" since 2011.
The Royal Navy website says anyone marked as MLD "requires a risk assessment to be carried out for each deployment".