Can't remember the source, but using the nasal cycle[1] as a source of randomness has been suggested before. The only drawback is the cycle length is on the order of 30 min to 6h so drawing more then 1bit/day is probably infeasible.

[1] - https://www.rhinologyonline.org/Rhinology_online_issues/2018...

The conventional technique in poker is to glance at the seconds hand on your watch