Since I learned that many nightmares occur when the blood takes growth hormones and leave the brain to reach vital organs and cells in the body. The feeling of "void" start producing the worst scenarios in the mind and project them on the cingulate cortex (works like a screen in the brain), this is what we call nightmares.

Other way the brain does internal detox of suppressed and oppressed feelings that we had in our waking life is to use those feelings and pick out bad memories from the hippocampus (like a wardrobe of memories) mix them together to remove them for gonna be from your mind.

Another way nightmares are helpful is to make you feel grateful for your waking life and that things are not that bad after all.

Sometimes nightmares are our intuition trying to bring our attention on something, someone that might be toxic and our conscious mind didn't realize that about them, or something we need to fix.

If you know these ways the brain function to protect you, you'll feel relieved after you wakeup from a nightmare.

What I do? Sometimes I turn tv on, sometimes I start tidying up my wardrobe, walk from one room to another, if it's already time I normally wakeup at, I start my day but I do things slowly, and go easy on myself.