how do atheists explain nightmare

[BrianSoddingBoru4]

The reasons we have nightmares and dreams has nothing to do with deities or the supernatural.

Dreams and nightmares are the way our brains process information in a way that doesn't put us in danger.
We go through fears and concerns, investigate possible solutions to problems, etc, all while being immobile
and thereby unable to actually get into harm from those actions.

if you have nightmares, that only means you have fears that your mind is attempting to work through.

I’ve heard dreams described (I forget by whom) as ‘Your subconscious whispering in your own ear.’ Seems about right.

Boru