Hey Jasmine - great question! I'm certainly not an expert in this area, but I think this is actually something a lot of runners notice after marathons and ultras, even if they don’t always label it as the SNS. The body stays in a pretty heightened state for awhile after long efforts, and it can take a few days or up to a week or 2 for things to settle back down fully. Which is why people feel exhausted and still have trouble sleeping, or notice their heart rate staying a bit elevated. Most of what I've seen tends to help are the recovery basics: focusing on sleep, stress, and fueling. It's important to recognize the deficit our body is in afterwards, and I think it is very common that runners underestimate how challenged our bodies were and therefore underestimate how long it can take to get back to feeling 'normal'. For fueling it is important to eat regularly even if your appetite feels off/not hungry. Easy movement like walking or light spinning usually helps more than complete rest. I also like having people spend a little time each day doing something that feels low-stimulus, like getting outside and doing some slow breathing. Sleep can definitely be inconsistent for a few nights, which is pretty normal post ultra. This should typically settle within a few days - if not and the above doesn't help it'd likely be worth digging deeper.

@Jasmine Fowler yes, both could certainly increase the severity. Where HR zone would be equivalent to effort (if measured accurately - a lot of watches aren't super accurate). And a higher effort for longer would increase full body fatigue after, as well as digging a deeper whole to have to 'recover out of'. Hydration is certainly very important and something I've always personally struggled with with longer efforts - everyone is so different. Could be worth doing a sweat test to get an idea of how concentrated your sweat is (there's a significant variance in people's ranges) - although typically people have a pretty good intuition. If you think the hydration/fueling could be a significant factor for you it could certainly be worth working with a registered dietician who really understands ultras. One of my favs is Holley Samuel - her website is here but she also has tons of great free content on her IG and podcast. https://holleyfuelednutrition.com/