Ask HN: Data mystery declining fitness while increasing training

I posted this on X with some graphs, but perhaps the HN-crowd, presumably better capable of saying insightful things about data and correlations, might offer some clues on the following mystery in my personal health/fitness data (collected with WHOOP).

My average HRV (indicator of fitness) has declined over the past 3 months, while I have intensified my daily intensity of training over the same period.

The 'peak' in HRV was this summer when I used an Apple Watch to do my workouts - benefitting from realtime feedback, pushing intensity albeit keeping workouts perhaps a little shorter.

You can clearly see when I stopped wearing my Apple Watch and resumed doing my training rounds based on feeling and whoop's data. I like mechanical watches and have developed some kind of hate/love relationship with smartwatches. That was mid august, exactly when the decline in fitness set in.

Funnily enough my best performances (PR's, pace, etc) were during the summer months - so HRV does seem to reflect my perceived personal fitness.

Any expert views on this? Could exposure to sunlight possibly affect the HRV metric in any significant way? I am located on the nothern hemisphere, dealing with shorter daylight periods in the winter months. Am I training to hard?

The link on X has the two graphs embedded/included.

Thanks for your help!