I've been looking for an app like you described too. And I found it!!! It's TimerBar. I had a hard time looking for it cuz its selling point is that it's a timer on the status bar. But it fails to let readers know that it's also on the status bar on the lock screen. You can find it on Cydia.
P.S. I created an account on this discussion board just to reply to your post. I've been looking around for such an app because I'm observing my newborn's sleeping pattern.