Hey, could you try refreshing the page and letting it fully load before hitting Sleep again? Also, does anything else seem off, or is it specifically just the Sleep button that's not responding?
No idea how long "fully loaded" is, but this has happened when I was playing for hours and THEN tried to sleep. Surely the game had loaded fully after a few hours.
That said, I hit sleep, the screen turns grey, and then it shows the offline window to show how much time I had offline and sets me back as if I just imported my save, instead of sleeping.