I would also suspect depression. It can make you absolutely knackered all the time despite being physically healthy. That said, if you want to explore the hormonal route, see an endocrinologist.
Your girlfriend seems to be in a right tizzy over night time driving. Is her anxiety founded in reality, ie has she ever nodded off, or almost nodded off, while driving? If so, then she should look into practical steps to staying awake. This article has several tips: the one about
keeping the interior light on at night time is interesting.
www.wikihow.com/Stay-Awake-when-DrivingEdit: Don't forget my post! >_>