Our results — recently published in Journal of Applied Psychology— were quite clear: Using the camera was positively correlated to daily feelings of fatigue; the number of hours that employees spent in virtual meetings were not. This indicates that keeping the camera consistently on during meetings is at the heart of the fatigue problem.
Even more interesting to us was our finding that fatigue reduced how engaged employees felt, as well as reducing their voice in meetings. Turning cameras on is often encouraged because it is popularly seen to help with both of these challenges—engagement and having everyone be heard — so it was notable that our findings indicated that feeling fatigued due to camera use may be actually undermining these goals in some situations.
As someone who is on a lot of meetings every day, this was an interesting study to read. I personally do have my camera on for all meetings, and I don't notice a fatigue, but maybe I should try turning it off for a week and see if anything changes.