henry copeland:

Morning sun 99.9% correlates with improved sleep

The Pittsburgh Sleep Quality Index, the Demographic Data Form, and the Follow-up Form for Elderly were used for data collection, whereas the Pittsburgh Sleep Quality Index was used for sample selection. It was found that exposure to direct sunlight between 8 AM and 10 AM for 5 days seems to be effective in increasing the global sleep quality score (P < .001).
- www.ncbi.nlm.nih.gov