I wanted to use sleep functions of the Atmega328 (in power-down mode), in conjunction with Arduino’s software serial library: it turns out that all interrupt vectors are enabled in sleep mode.
So the workaround is to start the serial communication when needed with
softSerial.begin( BAUD_RATE)), then stop it with
softSerial.end(). Also, the
.end() can be just before going to sleep, and the
.begin() just after.