[G.A.S.]

In that perticular situation, the thermostat would be the first thing on the adjenda. although, Take a look at your over flow jug and see where the fluid level is according to run time of the engine.
An engine running for a half hour should be at the HOT mark, and sitting over night it would read cold. If for some reason it is low, fill and check toe fluid level in the radiator. this would be checked when the engine is cold.
If the overflow is empty, there is an issue to deal with.