Setting: Mild fantasy -- but nothing particularly relevant to my question.
Question: I have a 17-year-old male who has been running on the edge of his capabilities for a period of several weeks. He's keeping himself going through sheer determination and need, and eventually, he's going to just run out and collapse.
What I'm wondering, and I'm not quite sure how to search for, would be what the actual symptoms/physical issues would be that would drive his exhaustion. Sleep deprivation, probably malnutrition to some degree (no time for meals -- he's just been like grabbing an apple or a handful of nuts when he can), bucketloads of anxiety and stress... but what actually makes him pass out and crumple into a melodramatic heap? I don't need like hyper-technical language, but I do kind of need to know what's going to be wrong with him beyond just "total end of his physical and mental capabilities."
ETA: For clarification, I'm not talking about a mental/emotional breakdown -- I'm talking about actually physically passing out, and thus looking for physical symptoms/specific problems.
Thank you for all your suggestions! Right now, I'm thinking of a combination of low blood sugar, sleep deprivation, mild dehydration,