Genius! I theorized Land of Departure would be sleeping since a few months ago along with the fact he could be searching for the Chamber of Waking to find Ven to form the X-Blade again or something. But I couldn't find a sure reason how he would know about the Realm of Sleep, not to mention enter it. But your theory makes perfect sense and I would love to see that be confirmed!I would assume it is similar to the same reason why Xemnas was trying to get to the Chamber of Waking, but not.
OKAY. Dumping my random brain blast here.
So Xemnas would use the Chamber of Repose to sleep so he could try and find the Chamber of Waking, which is in the Land of Depature. We can assume LoD is sleeping while Castle Oblivion is the current world we see.
We've also seen Xemnas and A:SoD in the RoS. We know Xemnas would frequently access the Chamber of Repose.
Also, we see YMX/MF/whoevertheheckthisguyis in the RoS as well. What if MX is trying to access the Chamber of Waking. What if the Chamber of Waking is the Key to Sleep or whatever?