We did a dream feed before I went to bed only if it were at least 2 hours (or three, as he got older) since the last feed, and it worked well for us between about 6 weeks and 6 months. Then, as he could easily go longer between feeds, we stopped doing it, as it wasn't changing how often he'd wake me up in the night for another feed anyway. And I started going to bed earlier- 9.30ish, so it was rarely more than 3 hours since that last feed.
I was able to dreamfeed DS1 and DS2 from when they were newborns. As has been explained, I would just gently pick them up, put them on the breast immediately once I'd sat down, feed them and put them back down in their cot. All with lights off (just a dim nightlight) and no talking to them. They slept through the night from very early ages (3 months old and 3 weeks old, respectively - and this was their lead, not mine). For whatever reason I don't dreamfeed DS3. I have done it, and he will happily feed while asleep, but whether he gets a late night snack or not, he will still wake in the wee hours for a feed.
I'd agree with other comments, some babies will dream feed and some won't.
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