Workfree I imagine what will happen is that your body learns to produce damn all milk at night, but if you're gonna continue with formula at night it surely doesn't matter. What I don't know is whether it'll tip over into affecting your daytime supply. One MW said to me that it's the nighttime feed that regulates everything? But surely that sounds odd? You may have to do a bit lf Googling there.
We had a rough night. He was fussy from 3.30am on. He keeps falling asleep after only 5 mins on the boob at night and I can't bloody wake him! So then of course he wants another feed too often, and he gets pissed off and starts crying. Then he swallows too much air and starts screaming etc. 
At 5.30 DH woke up to me in tears, with screaming back-arching baby at my poor sore boob, and with my milk completely drenching his GroBag from all DS's messing around. DH insisted on giving him the bottle of EBM in the fridge but then of course after all that trouble DS had no interest. And DH took off his GroBag to change his nappy but pulled the zip the wrong way and broke it! I almost started sobbing over the stupid zip!
Then DS started another screaming fit and we fed him dentinox and everything and nothing worked. Around 6.30 DS finally fell asleep with my arm around him but as soon as I pulled my arm away he started to awake. So I didn't really sleep.
Then at 9am DH brought him downstairs and he drank 90ml of the EBM. Its just so much easier to get it into him with bottles!
So lots of hollow-eyed silence in this house this morning. Thank God DH wasn't working today.
Honestly my aim with the breastfeeding now is just getting to 3 months. I honestly don't know how people extend it to 2 years etc. Its too exhausting.