Cockatiel parents take turns sitting on the eggs with, usually, the hen sitting during the night and the male replacing her in the morning so she can feed, water, fly a bit, etc. So what yours are doing is a natural behavior for them.
Don't get a dimmer, simply put a full spectrum light on the ceiling fixture and turn it off when the sun is halfway down to the horizon. A dimmer would only decrease the intensity, it would not change the spectrum - and it's precisely the different spectral distribution that happens during twilight that sets the internal clock.





