Sunshine gives our bodies Vitamins and energy and without that sunlight our batteries "so to speak" are drained and not able to be recharged.
The sun is our battery charger and we need sunlight to not feel tired and drained.
Another thing that happens when the sun is shining on our optic nerve our melatonin secretion decreases and when that happens we feel more awake.
When it's cloudy out with little to no sunshine the melatonin secretion doesn't decrease and we feel tired as a result of that.