We already know that a precise range of wavelengths within daylight triggers a light-sensitive photoreceptor in the back of your eye, causing the body s internal clock to reset.
Those receptors are called "intrinsically photosensitive retinal ganglion cells" (or ipRGCs), according to Science Alert.
As you ripped aside the curtains to bask in the beams of sunlight this morning, a domino effect of chemical reactions made sure your biology stayed in time with the endless loops of day and night.