This is not a nightmare, it belongs to your cerebral cortex, and you didn't go into deep sleep. Still feel conscious. During sleep, the brain goes into deep sleep about three or four times a day. Go to bed at ten, twelve and two. Half past three. Or go to deep sleep at 4: 30. Each time is about 13~ 15 minutes. During this period, the frequency of your eyes moving back and forth in the orbit is three times that of your eyes moving back and forth in the orbit. Soon. The brain is thinking about many things. Delete some memories selectively. . . .
So it's not a nightmare. It's just that you didn't sleep well that night I didn't go into deep sleep!