The manas cannot distinguish between being inside the dream and outside it

The manas cannot distinguish between dream and waking states; sometimes it takes the dream as reality, and sometimes it takes reality as a dream. While asleep, the manas is awake and, relying on the tathāgatagarbha, can discern all dharmas. When it sees someone approaching and considers this matter to be important, the mind consciousness emerges and also discerns it. The manas knows that it is sleeping, yet gives the mind consciousness a false impression, so the mind consciousness believes it is in a dream. In fact, it has not figured out whether it is inside or outside the dream, and so gets up to take precautionary measures. The main reason is still that the manas has become frightened, and thus startles the mind consciousness into wakefulness; when afraid, the mind no longer discriminates between dream and waking states.