Active minutes are based on heart rate to judge intensity. I suggest first drill down on active minutes reading to see what time they were, then looking at your overnight heart rate graph to see if it was elevated for that long at that time. That wouldn't tell you why the elevated readings - if real or bad readings somehow - but is a place to start looking.
The best way to see heart rate graph I find is the fitbit.com dashboard. Click 'Dashboard' near top right of this page (if on PC). Then on wide graph with 2 drop-down menus at right, select heart rate and daily. You can see your heart rate at 5 minute intervals and display readings as drag cursor along graph.
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