HRV Monitoring Sampling Frequency

[Ikerentrena]

Hello,
I am trying to conduct a study in which I want to measure stress levels of users during a cognitive development session. In order to get reliable information from the test I need to get a fairly high sampling rate.
Do you know what is the highest sampling rate that can be achieved for metrics such as HR or HRV in fitbit?

[Gondwana]

HR is normally recorded every 3+ seconds. When the user is running an Activity app or equivalent (eg, when running), it goes down to 1+ seconds.