Is the correlation between sleeping too long and bad health actually because sleeping too long is actually causally upstream of bad health effects, or only causally downstream of some common cause like illness?
Afaik, both. Like a lot of shit things—they are caused by depression, and they cause depression, horrible reinforcing loop. While the effect of bad health on sleep is obvious, you can also see this work in reverse; e.g. temporary severe sleep restriction has an anti-depressive effect. Notable, though with not many useful clinical applications, as constant sleep deprivation is also really unhealthy.
Is the correlation between sleeping too long and bad health actually because sleeping too long is actually causally upstream of bad health effects, or only causally downstream of some common cause like illness?
Afaik, both. Like a lot of shit things—they are caused by depression, and they cause depression, horrible reinforcing loop. While the effect of bad health on sleep is obvious, you can also see this work in reverse; e.g. temporary severe sleep restriction has an anti-depressive effect. Notable, though with not many useful clinical applications, as constant sleep deprivation is also really unhealthy.