Does light affect sneaking?
Yes, the level of light that you are currently standing in affects how easily you are spotted while sneaking. See the wiki entry on sneaking for more information.
As for how much it affects you, it only comes into play for visual identification, so you can sneak up on someone in pure daylight if you are approaching silently from behind, but they'll be able to spot you from much further away if they are looking at you.
The general official answer is : ANY light source actually affect the sneaking effect.
Now, the technical answer is much more friendly: As you might already know it, stealth is separated into 2 things : Visual and Sounds. The Stealth skill affect both.
Sounds initially is based on your equipped armors and spells/enchantment such as Muffle. Some skills like the weightless perk of both armor types reduce its relative armors sound by half. (Light armors got it as 20% base and Heavy got it at 40% base). An enemy doesn't get fully alerted by sounds but higher sounds makes them more wary from a longer distance. If you're a stealth archer, sound might never actually bother you. Dagger stealth killer though have to check their sound output closely.
Visual is the final check when being detected or not. Initially, you got a base Visual Stealth value which is based on your stealth skill. Then ANY light sources on your give out a negative bonus onto your base visual stealth value... including equipping magics spells (though this one only drop it by around 4%-5%)! The way light affects your stealth is based on a precise spot on your character body which is the center point of their chest. That "spot" calculate how much light "hits" it. This means that having light onto your head, hand, should or feet doesn't do anything... but as soon as your chest is lighted, you should be wary unless you got >90 stealth skill and its related perks bonus on detection. Think of it as this :
Without any stealth perks, lights affect your stealth value by an equivalent of 0.25*L. Where L equals the total amount of light that shine on your "center torso point". Light value is calculated as an 8 bit value : Between 0 and 255. 0 is when NO light affect you at all (pitch black area) and 255 is when you're fully lighted. Without mods, there's almost NO pitch black area, but those mods which affect the lightning of caves (realistic mod) actually helps on the stealth unknowingly. So, if you're in a pitch black area with no stealth perk, the stealth value doesn't get affected, but if you're in a fully lite area, it drops by 63.75%. Note that even in broad day light, the value is around 200 and not 255. You mostly only reach 255 if you're in day light using a torch and a light spell. So, in other words, being in day light only affect your stealth by about 50%... but that's BEFORE you learn any stealth perk.
For example, the Stealth perk in the Stealth tree gives up to a bonus of 80% reduction in detection. This means that you 50%-60% visual detection is dropped by 80% so to 10%-12% if maxed. It's without considering the Stealth skill itself. That's why having maxed Stealth perks + >90 Stealth skill usually makes you able to move within 3m in day light without being detected IN FRONT and pretty much almost touch the NPCs IN FRONT while in night or an interior.
So, does light on your affect your stealth? Yes. Does it matter once you're almost a grand master at stealth and its perks (also the light or heavy armor weightless perk for sounds being dropped by 50%), not at that point anymore.