Re: This is unnecessary
If it's a PITA, they're doing it wrong.
French law allows the employees' available periods to be defined, and if the employee is working, say, 8 hours a day with flexible start and end periods then their email/IM availability could also be defined as those same hours.
So if you're a night owl and work late for 2-3 hours in the day and everyone knows it and is happy with it, you wouldn't be expected to reply when you're not working but other people are.