I'm only new here, and medically inept, so forgive me if something like this has been discussed elsewhere:
My understanding is that CSF in the brain is produced, circulated, and replenished multiple times a day, every day. However if the regular daytime 'wave' actions are just too small to efficiently cleanse out many toxins / waste, and the lion's share is happening while asleep (due to the more intense movements), then this surely opens up a number of possibilities.
In Alzheimer's / dementia the intensity of this cleaning action is maybe being suppressed slowly over time, but perhaps in M.E. it's switching off almost completely after a period of over-exertion, and therefore resulting in the onset of PEM (and that 'poisoned' feeling) the next day? The 'recovery' period from this PEM episode may then be the night-time CSF wave-action slowly returning back to near-normal duties.
Alternatively it may be possible that the wave action is normal, but the composition of the CSF fluid itself may be somehow compromised after over-exertion (due to some sort of inflammation in the region of the brain that manufactures it).
Or maybe the CSF is becoming contaminated somehow - perhaps bacteria from a leaky gut, or dormant virus, but directly into the lower spinal canal instead of the bloodstream? These toxins in the slower-moving spinal canal fluid might then mix with the circulating fluid of the brain at night when the person's in a horizontal orientation, then doing the damage (with the resultant long-term lack of regular CSF 'housekeeping' ability playing a part in at least some of the bizarre and diverse symptoms we're familiar with).
Personally speaking, excessive impact or stress to my own lower back in the past would have been a catalyst for PEM the next day (although general over-exertion, like aerobic exercise, and mental stress could also be a trigger).
Another thing - magnesium supplements seem to help some people to lower PEM severity, and CSF seems to contain magnesium. And caffeine - I read somewhere that caffeine has been advantageous in increasing CSF volume in patients who've had CSF leaks. It's thought there are circadian variations in CSF production too, which may contribute to the slightly improved mood and energy levels experienced by some patients in the latter part of each day.
Anyhow, just some thoughts.......