Hey guy's got a weird thing happening with my civic which doesnt seem normal..

At night when its cold, i can drive for 20 minutes straight not hitting any stops or traffic lights and my thermostat heat level is still at the very bottom...

If i hit a light or two it sometimes warms up as i idle (1 quarter high), but never the normal heat level i get during the day (a bit below half).

If the car is already warm at night from hitting stop lights etc, my gauge quickly goes down when i just continuously cruise.

Im thinking a temp sensor? If anyone has any advice and any idea on how much it would cost it would be appreciated.

Thanks