Could be getting time for a new alternator. First,  check the
alternator belt itself. If it is shiny, it might be slipping at low
rpms. If it looks okay, then get a volt meter and measure the volts at
your battery at idle and then at a high enough rpm that the lights go
out.

If the volts at the battery are only reading 13.2 (or less) at high
rpms, you aren't really charging your battery very well (and you may be
in for a new battery in addition to a new alternator.)