Having had to do the SMF-SFO flight many times in the past, the likelihood of delay is extremely high if there is even a small amount of fog or rain (which is very frequent in SFO). If you can't change your flight, I would either plan to be ready to go to the airport early enough to catch the 6am flight and wake up as if you were catching that flight to check to see if your flight is delayed or SFO is having any weather. If so, try to same day change or standby for the 6am flight. Failing that, if a delay is posted to your flight, be prepared to drive to SFO to catch your flight, but only do this after confirming with an agent that the rest of your itinerary would be protected. Hopefully though all will be on time and you can go back to sleep for an hour or two,

One other thought, if there has been a schedule change that shortened your connect time since you booked it, you may be able to persuade United to rebook you on the earlier SMF-SFO flight without paying a change fee. However, in my mind, if all goes to plan, 1hr 20 min is adequate assuming that you are able to walk at a reasonable pace. If not, consider asking for a cart, etc. as the domestic to international walk can be quite a ways depending on which gates you fly in/out of.