For my case, i will wake up when I felt full automatically or just before baby woke up.
Like you said, I was too tired to pump out. But there were times when I just have to pump out as the milk was dripping.
I tried to pump or latch every 3-4 hours to stimulate my body. The best part was after you finished breastfeeding, you can doze off immediately. What I did was, I lied down and fed her. I can continue sleeping.