I know it�s not really why you�re asking but your daughter needs to take control of her health. I�ve see what you are describing before and I have a strong suspicion that better nutrition could help. She needs to make sure she eats a diet which prepares her for her shift and carries a quick energy snack with her, like a cereal bar, to pep her up during her shift. 18 year olds sometimes eat all the wrong things and doctors are crap at giving nutritional advice.
I suggest she keeps a food diary for 2 weeks then pops in and shows it to a dietician at the hospital to see if they can help her.
Meanwhile, she needs to be sensible about what she does at work, engineering 5 minute sit-downs a few times during her shift . And if the job can�t be changed she may need to look elsewhere in the health profession, like a care home.
Hope this helps.