Cold exposure in the morning can lead to a temporary drop in HRV followed by an improved HRV score over 2-3 hours.

“if your recovery score is in the tanks and you walk outside and you jump in your 35 degree water and you get back out what's going to happen is your HRV score immediately afterwards I'm I'm talking Within seconds is going to be significantly compromised right in other words think about that remember a low HRV means High sympathetic I promise you cold water will put you in a high sympathetic Drive however and we've tested this pretty extensively um looking at HRV zero 15 30 60 90 all the way up to 180 minutes post and on average you will see your HRV score continue to rise after that and so well you have this immediate sympathetic response you will immediately then respond you know about 30 minutes on most people depends on the person though and that score will be improved for several hours afterwards”

“After a brief initial adrenaline spike, cold water immersion dramatically elevates parasympathetic nervous system activity, increasing HRV and promoting relaxation This boost in HRV lasts for hours, priming your body for deep recovery If timed properly (~1-3 hours before bed), a brief plunge can make falling asleep easier and improve sleep quality”

“But in the ensuing hours you’ll see a massive down regulation, potentially as much as a 20–60% increase in HRV.”

“Cold exposure could help promote sleep by enhancing parasympathetic activity—but the timing is crucial.”

“And when you go into the ice, whether this is anywhere between 30 to 50° for a minute to 5 minutes, you're generally going to get out of that ice and you will see sympathetic drive. Fight or flight is elevated like pretty consistently. That said, we plotted this every 15 minutes for up to three hours post, and you just continually see a rise in parasympathetic drive for up to three hours post.”

“So, I don't know when he's using it, but I would be I would I would be willing to strongly bet he is far more downregulated going to bed when he does the ice for that exact reason. You got to time it appropriately because again when you get in the ice and immediately somewhere between 15 to 30 minutes post ice exposure you'll be more sympathetically driven but after that for several hours in most of the people they were far more and I'm talking about like 20 to 50% reductions elevations rather in HRV meaning more parasitic for many hours and we stopped actually collecting the data at three hours so I don't even know how long that that thing lasts but I'm not surprised”