The main causes of anal distension in the middle of the night include hemorrhoids, perianal abscesses and rectal prolapse.1. The most common cause is hemorrhoids. The nucleus of the internal hemorrhoids in particular is constantly increasing in size, stimulating the surrounding rectal mucosa and thus triggering distension and pain in the anus. The main reason for hemorrhoids is the varicose veins around the anus and the formation of venous blood clots or thrombi around the anus, which cause a feeling of swelling and pain. 2. Rectal prolapse. It is mainly due to the stimulation of feces and bacterial infection, resulting in edema of the mucous membrane at the end of the rectum and detachment from the rectal tissue, resulting in painful swelling at the anus, and in serious cases, bulging from the anus. 3. perianal abscess. This is mainly due to acute inflammatory lesions at the end of the rectum, resulting in edema, ulceration and pus flow, which eventually causes local tissue distention and pain.