Next stores opened at 0600 this morning for the first day of their sale, which means their staff had to get up in the middle of the night on Boxing Day. I doubt they were given any choice about this.
Other retailers, e.g. M&S, Tesco & Sainsbury's opened at 0900. John Lewis /Waitrose doesn't open at all on Boxing Day.
I understand that this is the busiest time of year for retailers, and that retail staff know what they are signing up for, but surely Next's decision to open at such a ridiculous hour is both unnecessary and unreasonable?