Caribmike, I am used to do that trip to Higuey, you can take the "guagua" even at 3 am ... from 3 am every one hour they leave from Friusa station. From 7 am I think they start using the big buses, but before, you will have to take the small ones which you are used to see around this area.
So, if you really need to be in SD early and 7 am is too late for you to take the direct bus, take one little bus to Higuey at 3 or 4 am and in higuey take the SD guagua, which leaves every 30 mins during whole night-morning.
Cheers.