I was tested severely deficient. Didn't have any obvious symptoms either.

The main problem of a vitamin D deficiency is poor bone and muscle health, not something that would usually cause obvious symptoms.

One of the hallmarks is lower back pain. If you get this frequently for no explicable reason you should be looking to get your levels tested.

Sleeping well? Were you waking up refreshed in the morning?

A lot of programmers don't sleep well regardless of their vitamin d intake. Late work, blue light all day, not enough exercise, stress, etc.

Not exactly. However, I've been taking Vitamin D for a while now and I can't say that feel "more refreshed".

I would guess that it depends on what your current vit D level is. Also, depending on your genotype you might be a fast or slow metabolizer of vit D, which would also play a role in how big of a dose you would need.

I recommend the talks/interviews by Dr. Stasha Gominak a neurologist specializing in sleep.

I have done 1000 IU for a few months with no impact on chronic insomnia.

The recommendation is 5,000 to 10,000 iu. 1,000 is what the label on the bottle says.

According to whom? NIH says 600 IU for a normal adult (https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessiona...) and recommends a maximum daily intake of 4000 IU (https://ods.od.nih.gov/factsheets/VitaminD-Consumer/).

My doctor specifically prescribes 5000 IU as a maintenance dose and 15000 IU to people severely deficient. These are numbers he got from treating hundreds of deficient patients and testing his patients blood levels every 6 months. I imagine a lot less is needed if you are in the sun for a good portion of the day every day but for most of us who spend a decent amount of time inside it seems to be what’s necessary.

My experience correlates with your doctor's: I had to take more than 5000 IUs per day for about 3 months to reach the bottom of the normal range for 25-OH-vit D.

Did you read the open letter (original link of thread)? There is an entire paragraph and footnote with 2 refs talking about the problem with the current government guidelines.

I've been doing 4,000 iu/day of Vitamin D since March, also supplementing Zinc and C, and taking a standard multivitamin. My entire household got COVID in November and I never became symptomatic.