[woj-tek]

While there are studies on 25-OH-D metabolism impacting melatonin production, I can only find studies done with Multiple Sclerosis patients [1]. There are also studies showing that LOW levels of 25-OH-D may have negative impact on sleep [2][3][4]. While I am unable to present scentific evidence, I would like to state that for me - personally - taking vitamin D3 in the afternoon has similar effects to ingesting caffeine. I have no reason to believe that I might be suffering from Multiple Sclerosis. Without evidence you are free to ignore it as a placebo/nocebo effect. YMMV based on age, ethnicity, sunlight etc.

The "take with meal" instruction is common with substances requiring fat for absorption/bioavailability. Dietary fat increases vitamin D3 absorption [5].

[1] https://pubmed.ncbi.nlm.nih.gov/23665342/ The influence of vitamin D supplementation on melatonin status in patients with multiple sclerosis

[2] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4507736/ 25-Hydroxyvitamin D Concentration and Sleep Duration and Continuity: Multi-Ethnic Study of Atherosclerosis

[3] https://pubmed.ncbi.nlm.nih.gov/28652922/ The relationship between serum vitamin D levels and sleep quality in fixed day indoor field workers in the electronics manufacturing industry in Korea

[4] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5331570/ Association between Serum Vitamin D Levels and Sleep Disturbance in Hemodialysis Patients

[5] https://pubmed.ncbi.nlm.nih.gov/25441954/ Dietary fat increases vitamin D-3 absorption