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!