I think that you should probably ask clarifying questions, granola is sugary but it's totally normal to be tired after loading up on carbs. It's when it pushes into extreme exhaustion that you should be concerned. Eating a bowl of sugary granola and ending up tired afterwards is normal, carb load + sugar rush/crash.

That being said, sleep deprivation also contributes to a higher risk of diabetes - from your cells being less sensitive to insulin (resistant), raising levels of ghrelin (hungry hormone), lowering levels of leptin (hormone that makes you feel full). It also fucks with your pancreas.

All in all, everyone should be careful about sugar intake/beetus - but a multitude of factors can contribute to it. Now if they're eating an entire bag of granola every night... yeah I'd find a different way to load up on carbs.