Extended Data Fig. 4: Comparing distribution of RHR overnight data in Fitbit vs. Apple Watch.

To show the distribution of RHR data in Fitbit and Apple Watch, RHR overnight data points have been divided into 16 ranges (1-10, 10-20, etc.) and each bar depicts the total number of nights that falls into each group. Each participant is presented with a color. Note that for the majority of participants, for most of the nights, there are 300 to 500 RHR overnight data points (that is, almost minute resolution) in the case of Fitbit. However, the range differs considerably in Apple Watch; the reason is that Apple Watch takes heart rate and step counts readings with different resolutions based on the activities. Note that in the NightSignal algorithm, for each night, we use the RHR overnight data regardless of the amount of data. The reasons for not setting a threshold for the minimum amount of data points required are as follows: the first reason is that in most cases, even a very few data points are sufficient to get a proper average RHR overnight because we only consider HR records where the corresponding time interval (for example, few minutes) for step count is zero. The second reason for that is if we do so (for example, set the threshold to 40 data points), we will miss a significant number of nights (for example, first four bars). It is important to note that there were 15 participants who had data collected from both Fitbit and Apple Watch. With respect to the average RHR overnight that is used in NightSignal algorithm, except for one participant who had significant different values between the Fitbit and Apple Watch (one possible reason can be that the watch has been used by another person), for other 14 participants, the median delta between the Fitbit and Apple Watch data for joint nights (total of 1,006 nights) is only two bpm. Hence, despite the difference between the distribution of RHR overnight data points, there is no significant difference in the average RHR overnight point of view that is being used in the NightSignal algorithm between two devices. It is noteworthy to mention that unfortunately we did not have many COVID-19 positive cases with Garmin data to properly evaluate the algorithm for Garmin watches, and thus we did not include Garmin in our analysis.