Yes, you can do it - the standard heartbeat detector that's built in doesn't do this since it uses a autocorrelation approach, but you can still access the raw data and figure it out from there - although the data is very noisy whenever you move your hand so you'd have to put a lot of effort in to filter the data.
Of course you can get a decent idea very easily just by comparing the BPM values you'll get from the built in detector every few seconds and seeing if they change noticeably.
Thanks. I'm not sure comparing BPM values would work as the variability is fairly slight, and would be masked by reading multiple pulses in a row.
Going to carry on playing with my puck for a bit, but pretty sure I know what my next watch is going to be :-)
Apparently the variability between inter-heartbeat frequency is quite a good indicator of stress.
Do you know if its possible to access the data to this degree, or is it a matter of asking the heartbeat sensor for a heart rate and getting an answer?
(You'd need a kind of OnPulse watcher!)