Hey, I had this same problem and wondered if its cause of too much atmosphere ( u get a msg at 10,000,000 saying they complaining about smog, and everyone isn't working cause of fatigue? Maybe thats why? Tell me what u think
There is a simple solution to that. MONEY. First tax all of your colonists till they have no money left via tax collection. Then "graciously" give them 250$. Their happiness will instantly go back to 100% and they will resume normal activities.
I actually abuse of that method to steal money from my colonists and make them happy about it ^^
All of this while paying them nearly nothing (5$)