Being in a very dark environment at night can provide beneficial effects for mental health, even without sufficient daytime sunlight.

“I predict that soon we will internalize the benefits of night time darkness on mental health.”

“if you didn't get sunlight during the day then you would be especially well off being in a very dark environment at night and it's independent so don't worry yes it's always good to get that daylight but what that paper also teaches us is that because those things can be independent you can still get some benefit even if though you've not made it good on your daylight during the day getting that Darkness at night is still going to be beneficial”

“people who make it a point to get dark exposure at night aka the absence of light at night actually benefit even if they're not getting enough sunlight during the day”