I have had a similar problem with my ds, on the days my MIL looks after him(at my house). This has taken me months to work out. He always woke at 5-530 on the day after MIL looked after him. We tried shorter naps for a while, then leaving him to wake on his own from naps for a while, nothing made a difference. What I realised is that he was doing so much more with MIL, as she doesnt have anything else to do all day except play with him etc, that he was getting much more tired. So I tried putting him to bed earlier on these days (which seems weird as he usually sleeps longer for her in the day also!). Only 20-30 minutes earlier, and I got a normal wake up. I hope this helps, as it drove me nuts for months, and poor MIL had a real complex about it.