This arises from discussions in comments on the question: How can a party defend against the Dream Haunting action of a Night Hag? .
The wording could either it lasts the duration the Night Hag is in contact with the target, or as long as the Hag is touching someone who is asleep. Based on Rules as Written, does waking up the victim end the effect of a Nightmare Haunting attack of a Night Hag?