How to differentiate between psychological and organic erectile dysfunction?

Presence of early morning erections and erection during REM sleep (nocturnal erection) are suggestive of psychogenic erectile dysfunction as there is no anxiety. Whereas, a patient with organic erectile dysfunction (due to vascular or neurological causes) would not have erections even during sleep. Investigations, such as penile plethysmography and nocturnal penile intumescence (NPT) can be used to record nocturnal erections.