A reader is concerned she can’t pump enough milk for her baby during her afternoon at work, so she must pump during the night to make up for that deficit.
One of the major factors in milk production is the hormone prolactin. Even though prolactin is low in women during pregnancy, it is helping build the milk-making structures in the breast. Prolactin levels take a huge jump when the placenta is delivered, telling your body, “Hey, it’s time to start making lots of milk now!”
There are two facets of prolactin levels during breastfeeding: 1) the base prolactin levels in blood which have a circadian rhythm (vary by the time of day); and 2) prolactin surges which occur a half hour or so after suckling has begun.
1) Generally, prolactin production is highest between 2 and 6 am. That’s one reason you may wake up feeling like you could feed the world. Base prolactin levels are lowest in the afternoon.
2) After each feeding/pumping session, there is a surge of prolactin which tells your body to step up milk production to replace the milk just removed. (One author did say the surge was slightly less dramatic when pumping as opposed to feeding.) The prolactin level gradually goes down over the next 3-4 hours. In the early days of breastfeeding when baby is feeding more frequently than that, the prolactin surges blend together and help establish a bountiful milk supply.
It is pretty normal to have less milk in the afternoon and I don’t think there is anything that can be done to specifically make more milk at a particular time of day. Pumping extra at some time is a way to shift the milk you do make to a time your baby is more likely to need it.
Another tip: you may not need to be concerned about spacing out the feeds/pumping sessions at completely even intervals (therefore not necessarily have to get up in the middle of the night). Could you squeeze some extra pumping in during the morning or before you go to bed?