A condition in which uncontrollable episodes of sleep occur two or three times a day. It starts at any age and persists for life. The attacks, which usually last for 10–15 minutes, come on suddenly, usually at times when anyone might feel sleepy, such as after a meal or sitting in a bus, but they may occur at any time, such as when walking in the street. After some years, they are frequently associated with cataplectic attacks (see CATAPLEXY), when for a few seconds there is sudden muscular weakness affecting the whole body and the person may fall to the ground. The condition may be due to a lack of the brain chemical transmitter, hypocretin. The cataplectic attacks can be helped by the TRICYCLIC ANTIDEPRESSANT DRUGS, imipramine or clomipramine SSRIs and sodium oxybate, the latter available in the UK for a minority of patients with poor response to other medication and only after special agreement has been reached for the drug to be funded.

It was a rare complication of the vaccine administered in 2009–2010 in anticipation of a pandemic of H1N1 influenza. Familial narcolepsy is well recognised, with up to 12 per cent of members of some families experiencing the condition. A gene variant, HLA-DQB1*06:02 has been found to be associated with the disorder (see HLA SYSTEM).