Apparantly お早うございます & お休みなさい are used among family members but not こんにちは&こんばんは. At some work places people regard other workers as similar to family members and consequently avoid using expressions that are used with non-family members. お早うございます is used in place of こんにちは&こんばんは and hence used at any hour of the day or night.
(This comes from Nihongo notes 10 p48, published by the Japan Times, written by Osamu & Nobuko Mizutani. I have talked about this with people but not really experienced it although as foreigners we are perhaps less likely to be welcomed into the "family circle" so quickly with a less than usual aisatsu.)