I remember when DD was born and lost too much weight, I had to set my alarm for every 2 hours and 45 minutes, day and night. Every time it went off, I'd change her, breastfeed her, then feed her expressed milk and formula top-up from a cup or syringe (which took ages), then express for 20 minutes, then take my pump to a room down the corridor and sterilise it. A good night for me was 1 1/2 hours sleep.
I feel slightly nostalgic for tiny weeny scrunched-up newborn DD, but no way do I miss those nights.