I was also looking to sleep for less than a second to save some power.
(Sleeping for less than a second)
There is no official support in particle.
So I came up with this
void msSleep(unsigned long ms) {
unsigned long start = millis();
while(millis() - start < ms) {
__WFI();
}
}
which save some milli amps, but I do not know if it has some side effects for the rest of the system.