SlackBuildsOrg/system/sleepd

sleepd is a daemon to to put a laptop to sleep if it is not
being used or if the battery is low.

You'll need to add the following lines to /etc/rc.d/rc.local.
# Start sleepd
if [ -x /etc/rc.d/rc.sleepd ]; then
  /etc/rc.d/rc.sleepd start
fi

You should then add the following to /etc/rc.d/rc.local_shutdown.
# Stop sleepd
if [ -x /etc/rc.d/rc.sleepd ]; then
  /etc/rc.d/rc.sleepd stop
fi