I have tried exactly what you are saying, and it does not work. The game sync tells me that it requires a day to recharge.
EDIT: I've had my coffee, and now I feel great! Sorry this post sounded so confrontational. Was just my usual morning grumpyness. >< I don't know if it's a regional difference, or something you're doing differently, but that method just does not work on my game.
It does work, actually, though it's a little complex. There are two different lockout methods used on the C-Gear.
1) You can only put a Pokemon to sleep once per calendar day.
2) If a time change is detected, you must wait 24 hours before you can put a Pokemon to sleep.
So long as you don't change the time, you can do this:
23:30 Put a Pokemon to sleep
23:35 Start DW
23:55 Exit DW
23:59 Wake up the Pokemon
00:05 Put another Pokemon to sleep
00:10 Start DW again
00:35 Get kicked out of DW by the hour limit
I've done it before on both UK and Japanese copies of BW.
By the way, calendar day is defined by the games from the time set upon your DS console.