Nightmares during the day vs. during the night

For a long time I've gotten nightmares about a traumatic event that happened a long time ago. It's weird though, I only ever get these nightmares at night.
I have narcolepsy, which means I dream when I nap.
When I takes naps during the day, I never get these nightmares, it's only during the night.
The traumatic event didn't happen during the night though...

Just wondering if anyone had any ideas why that might be?

I don't think that there's enough time during a narcolepsy period for your unconscious to put together a good-sized dream about anything other than what has happened close to the period? I know when I fall asleep while watching TV, stuff that was on TV or what I had for dinner or something are the immediate dreams I have, not the more complicated stuff the unconscious would have to dredge up and develop? Think of when you are "triggered". The trigger is immediate but the "story" that tells you what the trigger is triggering is much more detailed and takes longer to understand. No point in your unconscious just triggering you, it is on your side and wants you to understand so you can work through the traumatic event.

Shorter sleep time as the above poster said as well as perhaps a change in your circadian rhythm during the day which causes differing levels of hormones to be excreted versus how much at night. Also, light exposure during the day may affect the type and quality of sleep you are getting.

Did the event occur at night?