Tully was sleeping 6-8 hours per night by the time he was 6 weeks old. We had a pretty consistent routine - we’d say “goodnight”, tell him we were in the next room, and put him down. He’d zone out on something and go to sleep. We were really happy he could self-sooth and get himself to sleep. By 3 months he was even sleeping for 10 or 11 hours; we thought we’d cracked it.
Then at 4 months everything changed. He would cry the moment we put him down. I don’t know what the trigger was. I’d gone back to work, we’d taken a pretty long trip, it could have been anything for all we knew. We went to the paediatrician, and he just said “Sorry guys, this is normal.” He said around this age they become more aware - of their own surroundings, of you, of whether you’re in the room. He said sleep regression was a pretty important stage and now was the time to start sleep training. We had to go back to square one, and he made it clear we should start right away. He didn’t call it the ‘cry it out’ method but it did involve crying, and it was awful.