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.)