Do you have a regular tool holder maintenance schedule? If not you want to be cleaning the tapers on a regular basis....I always did mine once a week.
Your tool holders should be clean and shiny as should be the inside of the spindle taper. If you can't see up into the taper use a flashlight and a mirror.
You can purchase a male cleaning tool for the spindle. Just spin the router at a low speed and insert the fabric covered male taper tool up into the spindle.
For the tool holders I think the most recommended cleaner is bronze wool (think steel wool but made from bronze). When that wouldn't get them clean we'd fall back on the less favorable fine Scotch Brite pads. 20 seconds or so of rotating the tool holder back and forth in your hand while encasing the taper end with the bronze wood does the trick.
If the problem persists some might say to use anti-seize compound but that makes a mess and I'd be a bit concerned about tool holder slippage. I've used it on shaper spindle tapers but those go about 1/2 to 2/3 of the speed of your router spindle.
All this being said I have to ask how cold the shop gets at night. With the problem only occurring when the spindle is cold you have to consider metal expansion and contraction. My money though is on a dirty spindle taper.
BH Davis