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Sleep vasovagal syncope is defined as loss of consciousness in a non-intoxicated adult occurring during the night (e. g. 10:00 pm to 7:00 am), in which the patient wakes up with pre-syncopal and abdominal symptoms (i.e. an urge to defecate) and losses consciousness in bed or immediately upon standing. There is no tongue biting or post-ictal confusion. There is usually a history of daytime vasovagal syncope and there seems to be a more pronounced fear of blood and medical procedures than in other syncope patients (Jardine et al., 2006b). Physical examination, ECG and EEG are within normal limits. The vasovagal reaction is thought to start while asleep (Krediet et al., 2004a;Jardine et al., 2006a), and continuing after waking up, hence the name. During syncope there may be a profound sinus-bradycardia (Krediet et al., 2004a). Vasovagal sleep syncope occurs at all ages.

Sleep vasovagal syncope is diagnosed by excluding beyond reasonable doubt the hereafter mentioned disorders (Jardine et al., 2006a).

Epilepsy is the foremost alternative diagnosis to consider, but can often easily be ruled out on clinical grounds. Complex partial, generalized tonic-clonic and myoclonic epilepsy may occur during sleep and can imitate syncope when causing cause sinus-bradycardia (Tinuper et al., 2001).  

There are a number of related conditions, including “'''abdominal epilepsy'''” and '''Panayiotopoulos syndrome''' (typically with vomiting) (Covanis, 2006), in which the associated clinical features are abdominal pain and confusion.  

Occasionally cardiac disorders may cause cardiac arrhythmias during sleep. Most of these are unlikely if the 12-lead ECG is normal, and in some patients long-term ambulatory ECG monitoring is required (Brierley et al., 2001).

Some patients with a diagnosis of defaecation syncope (see below) described abdominal and pre-syncopal symptoms that started simultaneously during sleep (Pathy, 1978;Fisher, 1979); there may be some overlap between this condition and sleep syncope (Jardine et al., 2006a).