Functional abdominal pain: This is a common condition characterized by recurrent abdominal pain that is not explained by other medical conditions. The nocturnal pattern and periumbilical location are consistent with this diagnosis, and the lack of association with activity or eating also supports it.

Gastroesophageal reflux disease (GERD): Although GERD typically presents with symptoms related to eating, some patients may experience nocturnal pain due to acid reflux. The periumbilical location is less typical, but possible.
Peptic ulcer disease: Similar to GERD, peptic ulcer disease can cause nocturnal pain, and the periumbilical location could be consistent with a duodenal ulcer.
Irritable bowel syndrome (IBS): IBS can cause abdominal pain that is not necessarily related to eating or activity, and some patients may experience nocturnal symptoms.

Intestinal obstruction: Although less likely, intestinal obstruction can cause severe abdominal pain that may be worse at night. It is essential to consider this diagnosis, as it requires prompt medical attention.
Appendicitis: While appendicitis typically presents with pain that migrates to the right lower quadrant, some cases may have atypical presentations, including periumbilical pain.
Pancreatitis: Pancreatitis can cause severe abdominal pain that may be worse at night, and it is essential to consider this diagnosis due to its potential severity.

Abdominal migraine: This is a rare condition that can cause recurrent abdominal pain, often accompanied by other symptoms such as headache or nausea.
Celiac disease: Celiac disease can cause abdominal pain, but it is typically associated with other symptoms such as diarrhea or weight loss.
Inflammatory bowel disease (IBD): IBD, including Crohn's disease and ulcerative colitis, can cause abdominal pain, but it is typically associated with other symptoms such as diarrhea or weight loss.