Here are some studies relating to vitamin D, bright light, and insomnia:
Plasma Vitamin D Deficiency Is Associated With Poor Sleep Quality and Night-Time Eating at Mid-Pregnancy in Singapore (2017)
Conclusion: A study of 1,152 pregnant women found that those who were vitamin D-deficient had higher incidences of poor sleep quality compared to women with sufficient vitamin D levels.
Conclusion: A study of 141 hemodialysis patients found that those with lower vitamin D levels experienced lower sleep quality compared to those with higher vitamin D levels.
Vitamin D and Actigraphic Sleep Outcomes in Older Community-Dwelling Men: The MrOS Sleep Study (2015)
Conclusion: In a study of 3,048 elderly men, those with lower vitamin D levels were found to have lower sleep quality compared to men with higher vitamin D levels.
Improvement of Pain, Sleep, and Quality of Life in Chronic Pain Patients with Vitamin D Supplementation (2013)
Conclusion: In a study of 28 U.S. military veterans suffering from chronic pain, vitamin D supplementation was found to be effective for improving their pain levels, sleep quality, and overall quality of life.
Randomized, DIM Light Controlled, Crossover Test of Morning Bright Light Therapy for Rest-Activity Rhythm Disorders in Patients with Vascular Dementia and Dementia of Alzheimer’s Type (1998)
Conclusion: This study found that bright light therapy (you know… LIKE SUNLIGHT) improved sleep and the sleep/wake cycle of elderly dementia patients.
Conclusion: Blind people need bright light exposure (or sunlight exposure) too, but they’re less likely than other people to go and get it and this can cause problems with their sleep/wake cycle. In this study bright light exposure was found to be helpful for suppressing melatonin in blind people, which is important for waking up your brain up, getting yourself on a good sleep/wake cycle, and reducing insomnia.