How do I index to speed up OVERLAPS?

I have a table shift with a start_time column and a stop_time column and
I do a lot of queries like:

    select * from shift
    where (start_time, stop_time) overlaps ($A, $B);

$A and $B are user-submitted values.

Anyhow, how do I put indexes on my table to make these queries run
faster?

Also, is there a better data type to use for ranges of time?

TIA

Matt