Circuit breaker for master BR and bath trip about once a month since moving into house 6 years ago. It usually happens late at night when few electrical items are on - small lamp, TV, cable box, alarm clock. No problem when lots of things going, like all of the same plus hairdryer, charger for phone, charger for medical device, additional lamp, vacuum.

The entire circuit, each outlet and junction box, would need to be opened and checked for loose connections, shorted connection, defective device.

Sounds like the needle in the haystack.

Odd that it only trips at night. Any chance there is something that gets activated only at night?

A good troubleshooting service electrician with various instruments would be needed to find this problem.

You don't say how old your house is, but it is not usually permitted to have a bedroom and a bathroom typically wired on the same circuit. There are exceptions if only the bathroom lights are connected to a bedroom IIRC.

But that said.

Is this breaker an Arc-Fault-Circuit-Interrupter or a standard breaker?

Since electric codes have required AFCI breakers for bedrooms since about 2002, it is possible that it is an AFCI type.

If it is an AFCI breaker, it is also likley that it is one of the 1st generations of this type equipment, and the 1st generations of these type breakers were known to nuisance trip for no apparent reason.

If this is one of the early 'branch/feeder' type AFCI breakers, you should consider changing it to the newest 'combination type' AFCI breaker.

These are not only more relaible than the older types, they also sense more arcing fault conditions than the older 'branch/feeder' types.

If you change this AFCI breaker and you still have problems, you will need to do as tkrussell says, because you have a fault condition somewhere that needs to be isolated and corrected.

Luck