How to stop barking at night?

We have 2 six month shih tzus. In last week or so they have started barking throughout the night, one starts the other joins in. It can be for several reasons, one of the kids went to the toilet upstairs (we sleep downstairs and they sleep with us), a car went past (our house looks onto the street), the washing machine is finishing its cycle, or for other reasons w I didnt hear. This barking session can go on for several minutes and nothing seems to calm them down. Last night we has 5 incidents and im very tired. They had an hours walk last night, they sleep on our bed, we reassure them theres nothing to be worried about. But no surehow much broken sleep i can take at this point. Thinking about getting one of those barkinging aids that emit a high frequency noise every time they bark. What else should i do? Its be 16 years since my last dog was a puppy and i dont remember going through this 😂