Don't know if it's of any interest, but I own a device which does what
you're after - a Suunto T6C which records R-R interval for every
heartbeat. It should easily do a whole night of recording that at
normal resting heart rate (IIRC it does about 8 hours at exercise
heart rates, including altitude, so may manage 24 hours or so of
resting rate).

I think Polar also do at least one model which does much the same
thing. Neither of these options are cheap, and nor do they involve
making anything yourself, but if you're just after a device to measure
each heartbeat they're a lot less trouble.

Chris
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