The reason why nocturnal diarrhea is a red flag is because nocturnal diarrhea can suggest that you or someone with nocturnal diarrhea has a more serious underlying condition.
Unlike regular daytime diarrhea, that is often a result of temporary issues like food poisoning or the stomach flu, etc, the nocturnal diarrhea can be a result of something such as inflammatory bowel disease or IBD, malabsorption disorders or even microscopic colitis, which can indicate an issue with how your body absorbs or secretes the fluids and the electrolytes at night.
Diarrhea, which wakes you up at night is considered to be an alarm symptom and suggest that you have an organic or chronic problem, rather than just a minor and short term issue.
Nocturnal diarrhea is also a key feature in several gastrointestinal diseases, such as inflammatory bowel disease, Crohn's disease and ulcerative colitis as well as microscopic colitis.
Nocturnal diarrhea is also often caused by secretory diarrhea, which occurs when your intestines secrete more fluid than they absorb.
This can be a result of chronic infections, fats that are not absorbed, certain medications or even various types of internal conditions.
Nocturnal diarrhea can also negatively impact the quality of your sleep, although it's the underlying cause of the nocturnal diarrhea that makes the nocturnal diarrhea a red flag.
Other red flags to look out for with nocturnal diarrhea are fever or night sweats, unexplained weight loss, blood in your poop, severe or persistent abdominal pain and diarrhea which occurs even when you have an empty stomach.