Carrier heating unit (gas). Why does my heat system shut down for hours and than start up again on it own?

It sounds like it is overheating during a long run on cold nights then locking out.

On some newer boards the lockouts caused by some failure will time out after a number of hours.

It could also be an ignition failure.

To test my guess and assumption, when it is not heating you need to turn the power to the furnace and then turn it back on. If it was in a lockout it should restart the heating sequence as soon as the tstat calls for heat, that might happen right away or not depending on the thermostat.

Also you may have an indicator light that gives failure codes when it is locked out that will tell which component is tripping it off. Some newer ones have a little viewport on the blower door panel to see the flashing LED. Count the short and long flashes and compare to the legend on the inside of the blower door or the outer panel. for example, commonly, one long and three short is a pressure switch not closing.

Good Luck.

PS, one other thought. Is this a condensing furnace? I had one that was draining slowly and would lock out at night when it would run a lot. The buildup of water would cause an ignition failure because the pressure switch couldn’t sense, it was plugged with water. Then later when the water drained it worked fine. That one took me several trips to diagnose because by the time I got there the water was gone.