I didn't know that you could set the fitbit to sleep mode. I thought you had to manually enter the time on the website. I'll be trying this out tonight.
I've been doing it the past few nights. What's nice is it allows you to see how restless you are in your sleep rather than just a time.
After 1 night, I know not a good sample set, I don't see a difference than entering times on the website. Of course, I was using an app (Sleepbot and later a personal Path) to mark when I was going to sleep; similar to setting the fitbit to sleep mode. I like that I don't have to use the extra app and manually log in the information though.