I know that artificial light (i.e. lights generated by lamps, torches, et cetear) affects sneaking, but I'm not sure if this applies to natural light (light generated by the sun) as well.
Does natural light affect sneaking?
According to the Elder Scrolls Wiki entry on Sneak (emphasis added):
Characters' and animals' ability to notice the Dragonborn is dependent on a large number of factors, including their Sneak skill, line of sight, the level of light on their current position, how heavy the sneaker's armor is, whether this armor is heavy or light, enchantments on the armor (muffle), spells cast (muffle), if weapons are drawn, if magic is equipped, how fast they are moving, and the effects of any active perks.
So, the time of day may not directly effect your ability to hide, but the ambient light in your current location does, and that can be directly effected by the rising/setting sun, full moon, etc.
Yes, I know that the evidence isn't flawless, but I am convinced, by my prior belief and this evidence, to move on unless someone has solid counter-evidence. I note, for science, that it's possible that I got lucky and fluked on the skill checks.