Mind Constantly Decreasing?

[OOrochi]

New player, but Is my mind supposed to be constantly decreasing?
I had a run with Levi where I had done what felt like a lot of things and made it into the city during the first morning, and it seemed like my mind was holding at a pretty constant level throughout. However, I recently started a run as Marina, and the mind of all of my party members was going down at what felt like a pretty quick rate. For reference, I have Marina, Abella, and 2 Ghouls with me.

Could this be due to having the ghouls? Or is it just a normal occurrence that I somehow didn't notice on earlier runs?

[Cullen]

Your mind passively drains whenever in the dark. Generally this means indoor areas, but when it's night time your mind is always draining. It's also a more noticeable problem with parties since multiple characters are losing mind but you'll generally have the same amount of supplies to keep them topped off.

[OOrochi]

Ah, must've just spent more time in dark areas during that run. Thanks!