Cardiogram cofounder here. I had a fever back in January and my sleeping heart rate spiked during those nights. It was interesting to see such a big change, so we decided to build a new Sleep BPM feature & annotation tools around it. Hopefully this helps users out there become more aware of how your body responds to the flu or other illnesses like covid-19, and how your heart rate responds to flu-like symptoms. One important note though - these features are not intended for diagnostics! Would love to hear if the feature is helpful - ask me anything!
I often use my sleeping heart rate to know when I am becoming ill. I had the flu about a month ago, and saw a spike of 9bpm at the peak of the fever. At least for me, this metric is a really good indicator of health.