A/F problem

can't figure out what it could be, my teg normally goes from the bottom of lean to the upper tip of rich, but now at night time the light on the meter will go away and come back but not make it to the rich line. maybe 2-3 lights from it. now at wot during the day with no lights on it will go about half way into the rich area and at night with the lights on it will only go to about the rich line. i don't think that it is the alternator because i also have a autometer volt meter in the car and it pulles 14 volts. not sure why this is dong this. the car never runs this lean and it hasn't had any mods done to it since the guage was installed. a fpr might fix the prob of it running lean at night but than it would run to rich during the day when my lights are off. kinda doesn't make much sends but hopefully someone can understand this thanks