I've been using the sleep monitor for almost a week now, and it's very consistently telling me that I don't sleep very well. I concur.
The list is shown in reverse order starting with the most recent day.
The first day (Tuesday), I used the iPhone app to set the mode to "sleep" and in the morning I told it I was awake. And everything worked as expected.
On Wednesday, I did the same thing, but nothing was logged. Not sure why though. Maybe I didn't wait long enough for the app to connect to the Flex.
Since then, I've been using the two second fast tap method to put it into and take it out of sleep mode.