Why is my girlfriend always tired?

Most times I come to visit she is always super sleepy and she says that she is getting a lot of sleep. What could be causing her to be so tired although she is getting the recommended amount of sleep per night. I know she helps her family out a lot and she isn’t on any medications. She sleeps about 6–7 hours at night and she would take a nap shortly after getting up in the morning. She says she sleeps better during the day then at night but she says she is not having any nightmares but has been recently waking up frequently for no apparent reason.