I was extremely sleepy like that too last year and I thought it was stress, depression, or even the meds I take for insomnia... But in the end it was iron deficiency. Poor eating habits, skipping meals because I slept through the whole morning and woke up not feeling hungry at all, etc. I'm in my early 20s and my ferritin levels were so low for my age the doctor wondered how I was even functional.
The pressure of the studies might have contributed to it, but the culprit for me sleeping 10h+ and feeling lethargic all day was nutritional, not my depression, but NUTRIENTS. It could be the case for Yuna too... She might have to get hospitalized if this goes on, to get supplementation and all, regardless if it's a vitamin or mineral.
Morinaga COULD go for the "she's depressed" route too, but it coming down to her fundamentally not taking care of herself AT ALL instead of it being something she can't control would be a good plot point and reflect Yuna's character a lot. Caring too much about others and other things instead of prioritizing herself and cultivating good habits... And this, too, could be the introduction to talking about her mental state.