For me, excessive activity during the day leads to disturbed sleep. The other day I did a very long bike ride and then couldn't fall asleep for hours. My nervous system simply couldn't calm down, in an unpleasant way. It felt like the exercise had stirred up something in the body.
This for me too. The more I rest during the day the better my sleep. My sleep issues kicked in ~4/5 months into the Long Covid rollercoaster, preceded by what I'll broadly call "brain stuff" and along with new onset migraines, headaches & head pains. The last 3 have resolved 5 years in but the sleep issues remain. Oh and there were also random things like needing to pee a lot up to 10 times a night in the months after Covid which obviously disturbed sleep at that point too.
The sleep issues started with hypersomnia in yr 1, and progressed quickly into bad insomnia... hours to fall asleep, sometimes waking for hours during the night, but mostly once I got finally got asleep was ok, but then woke too early, not enough sleep. Then even if I had enough sleep felt totally unrefreshed.
Also for the first few years, don't know if anyone experienced this & I know I don't have words to describe it, but the hours trying to go to sleep I was not just "exhausted" or "wired"... it was some indescribable state of unbearable torture. Maybe that was a version of PEM after the day's exertion while I was still learning what PEM was & didn't really have a clear concept of reducing activity levels as much I have now.
Then melatonin helped with initiating sleep but led to the main issue switching to waking for hours in the middle of the night. And still regardless if I sleep well, rarely feeling refreshed. I get maybe 2-4 good nights' sleep in a month, i.e. sleep that I wake from refreshed and also that doesn't take hours trying to sleep, or waking for hours during the night.
Really bad PEM: I may not sleep at all the night of instigating it. This happened after my worst PEM episode - a Dr appointment that could have been online / telephone, I wasn't well enough at all to go but no choice. Sleep was an utter train wreck for weeks afterwards (as well as the rest of the PEM nightmare) and took ? 6 months to get back to "usual" after that appointment.