Heat Pumps and Electric Heating - heat pump throws circuit breaker

The heat pump works great until the outside temp gets below freezing. Then for some reason, it will throw the breaker. I wake up freezing, go downstairs, and flip the breaker back on. It will usually work fine after that. This happens always at night. Any ideas on what it could be?
Also, what is the difference in the heat running normal, aux. heat, and emergency heat?

the outside temp gets below freezing. Then for some reason, it will throw the breaker. I wake up freezing, go downstairs, and flip the breaker back on.
Out of the box here not there. But it could be that it tried to go into defrost so dont know if the defrostboard or the 4 way valve could be it. As I say not for sure have to check it all out with a meter.
what is the difference in the heat running normal, aux. heat, and emergency heat?
Normal will be the outside unit running with the inside fan.
if like it cant heat the home then the Aux heat (the electric elements will turn on) to help.
When the unit goes into defrost mode it is a AC again so the elements will come on so as not to blow cold air in the home.
emergency heat::: That will turn off the outside unit and run the electric elements to heat the home . It would be like you have just an electric furnace. At that time

" Out of the box here not there. But it could be that it tried to go into defrost so dont know if the defrostboard or the 4 way valve could be it. As I say not for sure have to check it all out with a meter."

Not sure what any of this means...so I guess it's time to call the heat/air guy!

You might check the breaker see if it gets warm in the day
time. With some H/P as it get colders at night . They run like all night long. This can trip the breaker SOME TIMES