You set the range of hours you want to have tracked for this. Tap profile pic at top left to get to 'Account' page; scroll down to GOALS - Activity > Hourly Activity Goal, and you can choose from 5-14 consecutive hours in the day to count. Steps outside of this range do not count for the hourly activity goal.
You can read more about it here:
What's my Fitbit hourly activity goal?
Best Answer