Writer - hope I can explain.
Yes, formula does have an impact on supply. In the early weeks and months of breastfeeding, round-the-clock stimulation of the system is needed to ensure sufficient milk. The milk needs to be removed often and effectively because milk that's made and which is not removed suppresses the supply - you can read a fuller explanation at www.kellymom.com, but it's basically to do with full breasts sending a 'message' to the pituitary to 'stop making milk'.
Long gaps between breastfeeds decrease the supply of milk, and while this is fine with very well-established breastfeeding (bf that has been going on well for many months), with a baby who is taking solids and who does not rely on breastfeeding, and who in any case may take less, it may not be fine with early breastfeeding. The production line tends to slow down and stop altogether if it's not stimulated.
Now, some mothers find their supply still remains even if they have introduced formula right at the start - they keep that single bottle as part of their routine, and everything still works just fine with the breastfeeding. In your own case, it sounds like you have a gap of maybe 5-6 hours once a day when the baby has formula, and your bf remains sufficient. For other women, this would just not work.
It's especially likely not to work if formula is introduced as a response to worries about milk supply - the baby has formula, the baby sleeps a long time, the mother thinks 'I'll give him some more as this proves my breastfeeding is not enough' and in addition to the impact of this one bottle, she adds further impact with another one.
The breast only makes 'as much milk as required' if the baby is enabled to feed often and effectively. Once you start messing about with this system, all bets are off.
Some women do manage fine, and maintain bf, despite the formula, but you can't predict which ones this will apply to.
As I say, in your case, you have a good reason for ensuring you have adequate rest, and the gap between your evening breastfeeds is not making a difference to the fact your baby breastfeeds the rest of the time - not everyone would be sure of this happening, though.
Hope this helps!