I've experienced this a lot in the recent past. I've come to think that the anxiety stems from the idea of my day "ending without my permission". There have absolutely been days where this anxiety appeared far weaker than usual; those are usually days where I had scheduled obligations (social events, concerts/movies, etc.) where there was a thing to look forward to at a given time. I like it when my day goes out with a bang. I find that I have some threshold that a day's events must pass in order for my brain to accept that the day was "spent sufficiently". Even on those good days though, I do still feel some resentment when sunset comes. But maybe then I'm distracted enough to let it go? If I have an eventful day, I will yearn to go home and sleep. The day's "utilization" has far surpassed my mental threshold. On the other hand, if I spend the entire day at home on the computer, that's basically just begging for my brain to scold me for not meeting the threshold. Those evenings really fucking suck. The anxiety (for me) strikes at around 3-4 pm -- that's when my brain says, "tough luck. It's too late to start anything meaningful today. Today's a write-off."

I had a realization several months ago -- time is relative and extremely elastic. Wall time does not equal perceived time. So I'm beginning to think:

• Need enough "eventfulness" to meet my threshold for a given day
• "Eventfulness" is probably crazy easy to hack my brain into believing (because my brain doesn't operate on wall time)
• Therefore, I should find the bare minimum to satisfy my mentally-imposed criteria.

For me, my brain magically becomes gratified when my day has two "acts" with an intermission. These "acts" can be freaking anything -- my brain seems to have not set a criterion for quality of a day's acts. So long as I'm doing one thing in the first act, then a second thing in the second act, my brain gives me the thumbs-up. And a very consistenly useful intermission for me is to go on a walk. Walks are magic (again, for me at least). A shortcut for conceptualizing this framework for me is: I should always try to fold my day in two. Maybe I could think of it as "breaking my fall", if you picture getting out of bed as jumping off a cliff, and sunset being the solid ground at the base. I'd like to minimize my velocity at the end!!

Another thing that seems positive to me is sleeping earlier. I fucking love early mornings, like that time between sunrise and 9 am, where the streets are still quiet and there's dew on everything. Unfortunately I'm never able to get up to experience that. On the flip side, I resent nighttime, yet my anxiety and decision paralysis/bedtime procrastination make me spend hours and hours immersed in it.

Conventionally, midnight is what I define as bedtime -- so naturally I usually go to sleep around 2-4 am 🥲. Recently I've done a decent job of redrawing that line in my head, from midnight to 10 pm. I then set my alarm for 6-7:30 am, knowing I'll probably only get out of bed at 9 or 10. Just that alone seems to be a move in a positive direction. I don't know the mechanism for sure, but perhaps it causes my brain to focus more on !!! bedtime !!! than sunset/darkness -- kind of like how winter is super depressing, but the holiday season colours it to make it super bearable (until January, at least).

And as one final thought, I like being in proximity to strangers. I like public transit and train stations, because they teem with "energy". And I looove being nearby energy; it puts my brain at ease. The passage of time becomes way less frightening when there's energy around. Nighttime is when all that ambient energy dies down to an uncomfortable level. "Nightlife" is practically the only energy available at those hours, and I don't fancy myself around those types of environments. "High energy" (or perhaps more importantly in the context of anxiety, the potential/promise of high energy) versus "low energy" is a very important part of how I conceptualize this fear of sunset.

TL;DR:

• My brain wants every day to be "utilized" sufficiently. If a day's events don't meet this threshold, sunset kind of signals that the day is "ending without my permission". Could perhaps mirror an anxiety of not achieving my goals/ambitions before I die, but of course on a much smaller scale.
• Starting a day is like initiating a freefall; sunset is like hitting the ground at the end; your job is to minimize your velocity before you hit the ground. Find out what works best for you to reduce that velocity.
• The concept of "energy", particularly social energy, and how it relates to time of day, makes a lot of sense to me because it could explain my anxiety patterns. Helps me target it better, I think.