It doesn't happen to me but a number of people reported that initially their sleep data didn't show up but then it did after a delay or a reboot of their phone. This seemed to happen to them particularly when they didn't sync every day. However, it seems that the data is there immediately on the web site which makes sense since it is calculated in Connect - not the vivofit. Are you checking on the web?
What I suspect is that, when this happens, clearing the cache on the app will force the sleep data to appear in the app but it's just a theory.
