I think sunlight makes us sleepy because it is comforting and sleep is the natural outcome of being comfortable. Difficulty with sleeping is caused by discomfort.
In that great masterpiece of literature, Tristram Shandy, the abbess of Quedlingberg, accompanied by her prioress, her deaness, her sub-chantress and senior canoness, who had journeyed from their cloisters to the university to consult the experts there upon a case of conscience relating to their placket holes, were up all night, tumbling and tossing, and tossing and tumbling, from vespers to matins, because the dimensions of the courteous stranger's nose had "got perched upon the top of the pineal gland" of their brains. This scene is laid in Strasburg where decent sunshine is not all that common.
The same effect can be acheived by under and over electric blankets. I don't think it is anything to worry about. I would be worried if sunshine and warmth made me jumpy and ready for action.
Maybe the problem is due to taking the Protestant work ethic too seriously.