I have lived in Japan for three weeks, and my HRV has noticeably improved.

After researching AI, I found that environments that are relatively cold and have stable daily routines are more conducive to the recovery of the autonomic nervous system.
With this in mind, my priority for retirement destinations is Japan, Northern Europe, and Australia.

PS: Long-term improvement in HRV often results from the combined effects of lower environmental temperatures + stable circadian rhythms + increased daily low-intensity activities + enhanced psychological security.