Hi
Personally set to "Normal". To help deciding which setting to use,
From the fitbit article:
How do I track my sleep with my Fitbit device?
What is the difference between the normal and sensitive sleep setting?
Your device's setting can record your sleep in either "sensitive" mode for extremely detailed sleep reporting or "normal" mode for a more basic idea of your sleep patterns. Note that if you use a Fitbit device with heart-rate tracking to detect your sleep stages, this setting doesn't apply.
Hope it helps