Thread: Affect of varying light on point of impact with open sights

From memory varying light does have an affect point of impact. Case in point Baikal with peep sight shooting 4" high at 100 meters in sunny conditions. Then in heavy overcast shooting spot on. I fired three groups to verify what I was seeing. Each was 3" in middle of black. I remember my number 4 did exactly the same. Unfortunately that was some years ago and I didn't make notes. Am I correct with this rule of thumb ; bright light aim low?
Cheers