In the darkness of the cupboard under the stairs, it was quite difficult to identify. While in the beginning Harry tried to orientate on his dreams, but the more time passed, the more their periodicity messed up. (And they become more intensive, but it’s a different story).The one reason why is because, in the "room" where he is now, there is actually nothing to do to become tired and have a natural sleep and moreover there is no light to help maintain the internal clock. Uncle Vernon ensured that: he has shut even the smallest hole in this space. He Xuan even wonders how there is something to breathe inside.