Notre Dame Tower at Night

That is only from 7th of July to 26th of August so have never experienced it at night.

Keep in mind sunset in the summer can be around 10 PM (22:00) so assuming they close at 11, likely you probably would be admitted in and climbing before sunset. Last admission probably at sunset time.
Basically your walk down would be after sunset but not a period of total darkness.

Have no clue about the lines, I want to think they would be much shorter since more folks will be having dinner, walking about, families going to bed, on a boat river cruise. Plently of likely better things to be doing so good chance the step climb is not that crowded. The lines/crowding were a bigger issue than the inside steps for me, so really don't think it would be dangerous in the dark interior.
No tripods allowed and the areas for picture taking you are usually rushed through, so would be hard to take a good photo after dark or even at sunset up there.

There are machines outside the doorway, which allow you to judge the time you wait.
Use "JeFile" - the app.
I don't know that you will see much but pretty lights...

"I don't know that you will see much but pretty lights.."

That may be the point of going up at night. The views of the lights of Paris from anywhere up high are wonderful, which is why the Eiffel Tower and the Arc d'Triomphe are so popular after dark.

Thanks for these responses. I am primarily trying to avoid the lines. Good idea about checking the wait time at the machine. I was thinking about going up at 7 or 8 to still see the sights but not have to wait so long. I did not realize it would be light as late as 10PM so that's good to know.

Hi spencer, here’s some info on sunrise/sunset in Paris:

https://www.timeanddate.com/sun/france/paris